Clarify our OOP semantics across the codebase (#9586)

Basically:

* Use `extend` for class definitions
* Use `new` for object instantiations

That includes some minor code cleanups along the way:

* Updated `Widget`'s docs to make the semantics clearer.
* Removed `should_restrict_JIT` (it's been dead code since https://github.com/koreader/android-luajit-launcher/pull/283)
* Minor refactoring of LuaSettings/LuaData/LuaDefaults/DocSettings to behave (mostly, they are instantiated via `open` instead of `new`) like everything else and handle inheritance properly (i.e., DocSettings is now a proper LuaSettings subclass).
* Default to `WidgetContainer` instead of `InputContainer` for stuff that doesn't actually setup key/gesture events.
* Ditto for explicit `*Listener` only classes, make sure they're based on `EventListener` instead of something uselessly fancier.
* Unless absolutely necessary, do not store references in class objects, ever; only values. Instead, always store references in instances, to avoid both sneaky inheritance issues, and sneaky GC pinning of stale references.
  * ReaderUI: Fix one such issue with its `active_widgets` array, with critical implications, as it essentially pinned *all* of ReaderUI's modules, including their reference to the `Document` instance (i.e., that was a big-ass leak).
* Terminal: Make sure the shell is killed on plugin teardown.
* InputText: Fix Home/End/Del physical keys to behave sensibly.
* InputContainer/WidgetContainer: If necessary, compute self.dimen at paintTo time (previously, only InputContainers did, which might have had something to do with random widgets unconcerned about input using it as a baseclass instead of WidgetContainer...).
* OverlapGroup: Compute self.dimen at *init* time, because for some reason it needs to do that, but do it directly in OverlapGroup instead of going through a weird WidgetContainer method that it was the sole user of.
* ReaderCropping: Under no circumstances should a Document instance member (here, self.bbox) risk being `nil`ed!
* Kobo: Minor code cleanups.

frontend/ui/widget/virtualkeyboard.lua

@@ -31,7 +31,7 @@ local DEFAULT_LABEL_SIZE = 22
 
 local VirtualKeyPopup
 
-local VirtualKey = InputContainer:new{
+local VirtualKey = InputContainer:extend{
     key = nil,
     icon = nil,
     label = nil,
@@ -456,11 +456,11 @@ function VirtualKey:invert(invert, hold)
     self:update_keyboard(hold, true)
 end
 
-VirtualKeyPopup = FocusManager:new{
+VirtualKeyPopup = FocusManager:extend{
     modal = true,
     disable_double_tap = true,
     inputbox = nil,
-    layout = {},
+    layout = nil, -- array
 }
 
 function VirtualKeyPopup:onTapClose(arg, ges)
@@ -489,6 +489,7 @@ function VirtualKeyPopup:init()
     local key_char_orig = key_chars[1]
     local key_char_orig_func = parent_key.callback
 
+    self.layout = {}
     local rows = {
         extra_key_chars = {
             key_chars[2],
@@ -726,21 +727,21 @@ function VirtualKeyPopup:init()
     end)
 end
 
-local VirtualKeyboard = FocusManager:new{
+local VirtualKeyboard = FocusManager:extend{
     name = "VirtualKeyboard",
     modal = true,
     disable_double_tap = true,
     inputbox = nil,
-    KEYS = {}, -- table to store layouts
-    shiftmode_keys = {},
-    symbolmode_keys = {},
-    utf8mode_keys = {},
-    umlautmode_keys = {},
+    KEYS = nil, -- table to store layouts
+    shiftmode_keys = nil, -- table
+    symbolmode_keys = nil, -- table
+    utf8mode_keys = nil, -- table
+    umlautmode_keys = nil, -- table
     keyboard_layer = 2,
     shiftmode = false,
     symbolmode = false,
     umlautmode = false,
-    layout = {},
+    layout = nil, -- array
 
     height = nil,
     default_label_size = DEFAULT_LABEL_SIZE,
@@ -784,11 +785,11 @@ function VirtualKeyboard:init()
     local lang = self:getKeyboardLayout()
     local keyboard_layout = self.lang_to_keyboard_layout[lang] or self.lang_to_keyboard_layout["en"]
     local keyboard = require("ui/data/keyboardlayouts/" .. keyboard_layout)
-    self.KEYS = keyboard.keys
-    self.shiftmode_keys = keyboard.shiftmode_keys
-    self.symbolmode_keys = keyboard.symbolmode_keys
-    self.utf8mode_keys = keyboard.utf8mode_keys
-    self.umlautmode_keys = keyboard.umlautmode_keys
+    self.KEYS = keyboard.keys or {}
+    self.shiftmode_keys = keyboard.shiftmode_keys or {}
+    self.symbolmode_keys = keyboard.symbolmode_keys or {}
+    self.utf8mode_keys = keyboard.utf8mode_keys or {}
+    self.umlautmode_keys = keyboard.umlautmode_keys or {}
     self.width = Screen:getWidth()
     local keys_height = G_reader_settings:isTrue("keyboard_key_compact") and 48 or 64
     self.height = Screen:scaleBySize(keys_height * #self.KEYS)