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/apps/reader/modules/readerzooming.lua

@@ -14,9 +14,9 @@ local Input = Device.input
 local Screen = Device.screen
 local T = require("ffi/util").template
 
-local ReaderZooming = InputContainer:new{
+local ReaderZooming = InputContainer:extend{
     zoom = 1.0,
-    available_zoom_modes = {
+    available_zoom_modes = { -- const
         "page",
         "pagewidth",
         "pageheight",
@@ -27,26 +27,26 @@ local ReaderZooming = InputContainer:new{
         "rows",
         "manual",
     },
-    zoom_genus_to_mode = {
+    zoom_genus_to_mode = { -- const
         [4] = "page",
         [3] = "content",
         [2] = "columns",
         [1] = "rows",
         [0] = "manual",
     },
-    zoom_mode_to_genus = {
+    zoom_mode_to_genus = { -- const
         page    = 4,
         content = 3,
         columns = 2,
         rows    = 1,
         manual  = 0,
     },
-    zoom_type_to_mode = {
+    zoom_type_to_mode = { -- const
         [2] = "",
         [1] = "width",
         [0] = "height",
     },
-    zoom_mode_to_type = {
+    zoom_mode_to_type = { -- const
         [""]   = 2,
         width  = 1,
         height = 0,
@@ -58,7 +58,7 @@ local ReaderZooming = InputContainer:new{
     -- with overlap of zoom_overlap_h % (horizontally)
     -- and zoom_overlap_v % (vertically).
     kopt_zoom_factor = 1.5,
-    zoom_pan_settings = {
+    zoom_pan_settings = { -- const
         "kopt_zoom_factor",
         "zoom_overlap_h",
         "zoom_overlap_v",
@@ -71,7 +71,7 @@ local ReaderZooming = InputContainer:new{
     zoom_direction_vertical = nil, -- true for column mode
     current_page = 1,
     rotation = 0,
-    paged_modes = {
+    paged_modes = { -- const
         page = _("Zoom to fit page works best with page view."),
         pageheight = _("Zoom to fit page height works best with page view."),
         contentheight = _("Zoom to fit content height works best with page view."),