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/overlapgroup.lua

@@ -3,9 +3,10 @@ A layout widget that puts objects above each other.
 --]]
 
 local BD = require("ui/bidi")
+local Geom = require("ui/geometry")
 local WidgetContainer = require("ui/widget/container/widgetcontainer")
 
-local OverlapGroup = WidgetContainer:new{
+local OverlapGroup = WidgetContainer:extend{
     -- Note: we default to allow_mirroring = true.
     -- When using LeftContainer, RightContainer or HorizontalGroup
     -- in an OverlapGroup, mostly when they take the whole width,
@@ -36,19 +37,22 @@ function OverlapGroup:getSize()
     return self._size
 end
 
-function OverlapGroup:initDimen()
+-- NOTE: OverlapGroup is one of those special snowflakes that will compute self.dimen at init,
+--       instead of at paintTo...
+function OverlapGroup:init()
     self:getSize()  -- populate self._size
     -- sync self._size with self.dimen, self.dimen has higher priority
-    if self.dimen.w then
-        self._size.w = self.dimen.w
-    else
-        self.dimen.w = self._size.w
-    end
-    if self.dimen.h then
-        self._size.h = self.dimen.h
-    else
-        self.dimen.h = self._size.h
+    if self.dimen then
+        -- Jump through some extra hoops because this may not be a Geom...
+        if self.dimen.w then
+            self._size.w = self.dimen.w
+        end
+        if self.dimen.h then
+            self._size.h = self.dimen.h
+        end
     end
+    -- Regardless of what was passed to the constructor, make sure dimen is actually a Geom
+    self.dimen = Geom:new{x = 0, y = 0, w = self._size.w, h = self._size.h}
 end
 
 function OverlapGroup:paintTo(bb, x, y)