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/widget.lua

@@ -14,44 +14,42 @@ local EventListener = require("ui/widget/eventlistener")
 
 --- Widget base class
 -- @table Widget
-local Widget = EventListener:new()
+local Widget = EventListener:extend{}
 
 --[[--
-Use this method to define a subclass widget class that's inherited from a
-base class widget. It only setups the metabale (or prototype chain) and will
-not initiate a real instance, i.e. call self:init().
+Use this method to define a widget subclass that's inherited from a base class widget.
+It only setups the metatable (or prototype chain) and will not initiate a real instance, i.e. call self:init().
 
-@tparam Widget baseclass
+@tparam table subclass
 @treturn Widget
 ]]
-function Widget:extend(baseclass)
-    local o = baseclass or {}
+function Widget:extend(subclass_prototype)
+    local o = subclass_prototype or {}
     setmetatable(o, self)
     self.__index = self
     return o
 end
 
 --[[--
-Use this method to initiatie a instance of a class, don't use it for class
-definition because it also calls self:init().
+Use this method to initiate an instance of a class.
+Do NOT use it for class definitions because it also calls self:init().
 
-@tparam Widget o
+@tparam table o
 @treturn Widget
 ]]
 function Widget:new(o)
     o = self:extend(o)
-    -- Both o._init and o.init are called on object creation. But o._init is
-    -- used for base widget initialization (basic component used to build other
-    -- widgets). While o.init is for higher level widgets, for example Menu
-    -- Widget
+    -- Both o._init and o.init are called on object creation.
+    -- But o._init is used for base widget initialization (basic components used to build other widgets).
+    -- While o.init is for higher level widgets, for example Menu.
     if o._init then o:_init() end
     if o.init then o:init() end
     return o
 end
 
 --[[
-FIXME: enable this doc section when we verified all self.dimen is a Geom so we
-can return self.dime:copy().
+FIXME: Enable this doc section once we've verified all self.dimen are Geom objects
+       so we can return self.dimen:copy() instead of a live ref.
 
 Return size of the widget.
 
@@ -64,8 +62,8 @@ end
 --[[--
 Paint widget to a BlitBuffer.
 
-@tparam BlitBuffer bb BlitBuffer to paint to. If it's the screen BlitBuffer, then
-widget will show up on screen refresh.
+@tparam BlitBuffer bb BlitBuffer to paint to.
+If it's the screen BlitBuffer, then widget will show up on screen refresh.
 @int x x offset within the BlitBuffer
 @int y y offset within the BlitBuffer
 ]]