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/document/document.lua

@@ -16,13 +16,15 @@ This is an abstract interface to a document
 local Document = {
     -- file name
     file = nil,
+    -- engine instance
+    _document = nil,
 
-    links = {},
+    links = nil, -- table
 
     GAMMA_NO_GAMMA = 1.0,
 
     -- override bbox from orignal page's getUsedBBox
-    bbox = {},
+    bbox = nil, -- table
 
     -- flag to show whether the document was opened successfully
     is_open = false,
@@ -40,10 +42,15 @@ local Document = {
 
 }
 
-function Document:new(from_o)
-    local o = from_o or {}
+function Document:extend(subclass_prototype)
+    local o = subclass_prototype or {}
     setmetatable(o, self)
     self.__index = self
+    return o
+end
+
+function Document:new(o)
+    o = self:extend(o)
     if o._init then o:_init() end
     if o.init then o:init() end
     return o
@@ -51,7 +58,9 @@ end
 
 -- base document initialization should be called on each document init
 function Document:_init()
-    self.configurable = Configurable:new()
+    self.links = {}
+    self.bbox = {}
+    self.configurable = Configurable:new{}
     self.info = {
         -- whether the document is pageable
         has_pages = false,