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/inputtext.lua

@@ -18,10 +18,10 @@ local util = require("util")
 local _ = require("gettext")
 local Screen = Device.screen
 
-local Keyboard
-local FocusManagerInstance = FocusManager:new{}
+local Keyboard -- Conditional instantiation
+local FocusManagerInstance -- Delayed instantiation
 
-local InputText = InputContainer:new{
+local InputText = InputContainer:extend{
     text = "",
     hint = "demo hint",
     input_type = nil, -- "number" or anything else
@@ -576,8 +576,7 @@ function InputText:onKeyPress(key)
         if key["Backspace"] then
             self:delChar()
         elseif key["Del"] then
-            self:rightChar()
-            self:delChar()
+            self:delNextChar()
         elseif key["Left"] then
             self:leftChar()
         elseif key["Right"] then
@@ -616,6 +615,9 @@ function InputText:onKeyPress(key)
         -- FocusManager may turn on alternative key maps.
         -- These key map maybe single text keys.
         -- It will cause unexpected focus move instead of enter text to InputText
+        if not FocusManagerInstance then
+            FocusManagerInstance = FocusManager:new{}
+        end
         local is_alternative_key = FocusManagerInstance:isAlternativeKey(key)
         if not is_alternative_key and Device:isSDL() then
             -- SDL already insert char via TextInput event
@@ -814,6 +816,16 @@ function InputText:delChar()
     self:initTextBox(table.concat(self.charlist))
 end
 
+function InputText:delNextChar()
+    if self.readonly or not self:isTextEditable(true) then
+        return
+    end
+    if self.charpos > #self.charlist then return end
+    self.is_text_edited = true
+    table.remove(self.charlist, self.charpos)
+    self:initTextBox(table.concat(self.charlist))
+end
+
 function InputText:delToStartOfLine()
     if self.readonly or not self:isTextEditable(true) then
         return
@@ -862,11 +874,13 @@ function InputText:goToEndOfLine()
 end
 
 function InputText:goToHome()
-    self.text_widget:moveCursorToCharPos(1)
+    self.text_widget:moveCursorHome()
+    self.charpos, self.top_line_num = self.text_widget:getCharPos()
 end
 
 function InputText:goToEnd()
-    self.text_widget:moveCursorToCharPos(0)
+    self.text_widget:moveCursorEnd()
+    self.charpos, self.top_line_num = self.text_widget:getCharPos()
 end
 
 function InputText:moveCursorToCharPos(char_pos)