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/readerthumbnail.lua

@@ -2,11 +2,11 @@ local Blitbuffer = require("ffi/blitbuffer")
 local Cache = require("cache")
 local Device = require("device")
 local Geom = require("ui/geometry")
-local InputContainer = require("ui/widget/container/inputcontainer")
 local Persist = require("persist")
 local RenderImage = require("ui/renderimage")
 local TileCacheItem = require("document/tilecacheitem")
 local UIManager = require("ui/uimanager")
+local WidgetContainer = require("ui/widget/container/widgetcontainer")
 local Screen = Device.screen
 local ffiutil = require("ffi/util")
 local logger = require("logger")
@@ -18,7 +18,7 @@ local _ = require("gettext")
 -- It handles launching via the menu or Dispatcher/Gestures two fullscreen
 -- widgets related to showing pages and thumbnails that will make use of
 -- its services: BookMap and PageBrowser.
-local ReaderThumbnail = InputContainer:new{}
+local ReaderThumbnail = WidgetContainer:extend{}
 
 function ReaderThumbnail:init()
     if not Device:isTouchDevice() then