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/device/sdl/device.lua

@@ -60,7 +60,7 @@ local external = require("device/thirdparty"):new{
     end,
 }
 
-local Device = Generic:new{
+local Device = Generic:extend{
     model = "SDL",
     isSDL = yes,
     home_dir = os.getenv("XDG_DOCUMENTS_DIR") or os.getenv("HOME"),
@@ -105,21 +105,21 @@ local Device = Generic:new{
     window = G_reader_settings:readSetting("sdl_window", {}),
 }
 
-local AppImage = Device:new{
+local AppImage = Device:extend{
     model = "AppImage",
     hasMultitouch = no,
     hasOTAUpdates = yes,
     isDesktop = yes,
 }
 
-local Desktop = Device:new{
+local Desktop = Device:extend{
     model = SDL.getPlatform(),
     isDesktop = yes,
     canRestart = notOSX,
     hasExitOptions = notOSX,
 }
 
-local Emulator = Device:new{
+local Emulator = Device:extend{
     model = "Emulator",
     isEmulator = yes,
     hasBattery = yes,
@@ -137,7 +137,7 @@ local Emulator = Device:new{
     canStandby = no,
 }
 
-local UbuntuTouch = Device:new{
+local UbuntuTouch = Device:extend{
     model = "UbuntuTouch",
     hasFrontlight = yes,
     isDefaultFullscreen = yes,