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/touchmenu.lua

@@ -39,7 +39,7 @@ local DGENERIC_ICON_SIZE = G_defaults:readSetting("DGENERIC_ICON_SIZE")
 --[[
 TouchMenuItem widget
 --]]
-local TouchMenuItem = InputContainer:new{
+local TouchMenuItem = InputContainer:extend{
     menu = nil,
     vertical_align = "center",
     item = nil,
@@ -267,9 +267,9 @@ end
 --[[
 TouchMenuBar widget
 --]]
-local TouchMenuBar = InputContainer:new{
+local TouchMenuBar = InputContainer:extend{
     width = Screen:getWidth(),
-    icons = {},
+    icons = nil, -- array, mandatory
     -- touch menu that holds the bar, used for trigger repaint on icons
     show_parent = nil,
     menu = nil,
@@ -423,7 +423,7 @@ function TouchMenuBar:init()
             self.bar_sep
         },
     }
-    self.dimen = Geom:new{ w = self.width, h = self.height }
+    self.dimen = Geom:new{ x = 0, y = 0, w = self.width, h = self.height }
 end
 
 function TouchMenuBar:switchToTab(index)
@@ -443,7 +443,7 @@ end
 --[[
 TouchMenu widget for hierarchical menus
 --]]
-local TouchMenu = FocusManager:new{
+local TouchMenu = FocusManager:extend{
     tab_item_table = nil, -- mandatory
     -- for returning in multi-level menus
     item_table_stack = nil,