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/apps/reader/modules/readerstyletweak.lua

@@ -19,6 +19,7 @@ local TextBoxWidget = require("ui/widget/textboxwidget")
 local UIManager = require("ui/uimanager")
 local VerticalGroup = require("ui/widget/verticalgroup")
 local VerticalSpan = require("ui/widget/verticalspan")
+local WidgetContainer = require("ui/widget/container/widgetcontainer")
 local lfs = require("libs/libkoreader-lfs")
 local logger = require("logger")
 local util = require("util")
@@ -27,7 +28,7 @@ local Screen = Device.screen
 local T = require("ffi/util").template
 
 -- Simple widget for showing tweak info
-local TweakInfoWidget = InputContainer:new{
+local TweakInfoWidget = InputContainer:extend{
     tweak = nil,
     is_global_default = nil,
     toggle_global_default_callback = function() end,
@@ -226,7 +227,7 @@ end
 
 -- Reader component for managing tweaks. The aggregated css_text
 -- is actually requested from us and applied by ReaderTypeset
-local ReaderStyleTweak = InputContainer:new{
+local ReaderStyleTweak = WidgetContainer:extend{
     tweaks_by_id = nil,
     tweaks_table = nil, -- sub-menu items
     nb_enabled_tweaks = 0, -- for use by main menu item