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/menu.lua

@@ -40,7 +40,7 @@ local T = FFIUtil.template
 --[[--
 Widget that displays a shortcut icon for menu item.
 --]]
-local ItemShortCutIcon = WidgetContainer:new{
+local ItemShortCutIcon = WidgetContainer:extend{
     dimen = Geom:new{ w = Screen:scaleBySize(22), h = Screen:scaleBySize(22) },
     key = nil,
     bordersize = Size.border.default,
@@ -88,7 +88,7 @@ end
 --[[
 Widget that displays an item for menu
 --]]
-local MenuItem = InputContainer:new{
+local MenuItem = InputContainer:extend{
     text = nil,
     bidi_wrap_func = nil,
     show_parent = nil,
@@ -425,7 +425,6 @@ function MenuItem:init()
 end
 
 local _dots_cached_info
-
 function MenuItem:getDotsText(face)
     local screen_w = Screen:getWidth()
     if not _dots_cached_info or _dots_cached_info.screen_width ~= screen_w
@@ -565,7 +564,7 @@ end
 --[[
 Widget that displays menu
 --]]
-local Menu = FocusManager:new{
+local Menu = FocusManager:extend{
     show_parent = nil,
 
     title = "No Title",
@@ -576,7 +575,7 @@ local Menu = FocusManager:new{
     header_padding = Size.padding.default,
     dimen = nil,
     item_table = nil, -- NOT mandatory (will be empty)
-    item_shortcuts = {
+    item_shortcuts = { -- const
         "Q", "W", "E", "R", "T", "Y", "U", "I", "O", "P",
         "A", "S", "D", "F", "G", "H", "J", "K", "L", "Del",
         "Z", "X", "C", "V", "B", "N", "M", ".", "Sym",
@@ -643,7 +642,7 @@ function Menu:init()
     self.show_parent = self.show_parent or self
     self.item_table = self.item_table or {}
     self.item_table_stack = {}
-    self.dimen = Geom:new{ w = self.width, h = self.height or Screen:getHeight() }
+    self.dimen = Geom:new{ x = 0, y = 0, w = self.width, h = self.height or Screen:getHeight() }
     if self.dimen.h > Screen:getHeight() or self.dimen.h == nil then
         self.dimen.h = Screen:getHeight()
     end