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/dump.lua

@@ -1,5 +1,7 @@
 --[[--
 A simple serialization function which won't do uservalues, functions, or loops.
+
+If we ever need a more full-featured variant, the consensus seems to be https://github.com/pkulchenko/serpent ;).
 ]]
 
 local isUbuntuTouch = os.getenv("UBUNTU_APPLICATION_ISOLATION") ~= nil
@@ -15,7 +17,8 @@ local function _serialize(what, outt, indent, max_lv, history, pairs_func)
         return
     end
 
-    if type(what) == "table" then
+    local datatype = type(what)
+    if datatype == "table" then
         history = history or {}
         for up, item in ipairs(history) do
             if item == what then
@@ -43,9 +46,9 @@ local function _serialize(what, outt, indent, max_lv, history, pairs_func)
             insert(outt, string.rep(indent_prefix, indent))
         end
         insert(outt, "}")
-    elseif type(what) == "string" then
+    elseif datatype == "string" then
         insert(outt, string.format("%q", what))
-    elseif type(what) == "number" then
+    elseif datatype == "number" then
         if isUbuntuTouch then
             --- @fixme The `SDL_CreateRenderer` function in Ubuntu touch somehow
             -- use a strange locale that formats number like this: 1.10000000000000g+02
@@ -55,11 +58,11 @@ local function _serialize(what, outt, indent, max_lv, history, pairs_func)
         else
             insert(outt, tostring(what))
         end
-    elseif type(what) == "boolean" then
+    elseif datatype == "boolean" then
         insert(outt, tostring(what))
-    elseif type(what) == "function" then
+    elseif datatype == "function" then
         insert(outt, tostring(what))
-    elseif type(what) == "nil" then
+    elseif datatype == "nil" then
         insert(outt, "nil")
     end
 end