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.

plugins/backgroundrunner.koplugin/main.lua

@@ -115,7 +115,6 @@ function BackgroundRunner:_shouldRepeat(job)
 end
 
 function BackgroundRunner:_finishJob(job)
-    assert(self ~= nil)
     if type(job.executable) == "function" then
         local time_diff = job.end_time - job.start_time
         local threshold = time.s(1)
@@ -159,7 +158,6 @@ end
 
 --- Polls the status of the pending CommandRunner.
 function BackgroundRunner:_poll()
-    assert(self ~= nil)
     assert(CommandRunner:pending())
     local result = CommandRunner:poll()
     if result == nil then return end
@@ -169,7 +167,6 @@ end
 
 function BackgroundRunner:_execute()
     logger.dbg("BackgroundRunner: _execute() @ ", os.time())
-    assert(self ~= nil)
     if CommandRunner:pending() then
         self:_poll()
     else
@@ -236,7 +233,6 @@ function BackgroundRunner:_execute()
 end
 
 function BackgroundRunner:_schedule()
-    assert(self ~= nil)
     if self.running == false then
         if #self.jobs == 0 and not CommandRunner:pending() then
             logger.dbg("BackgroundRunnerWidget: no job, not running @ ", os.time())
@@ -252,14 +248,13 @@ function BackgroundRunner:_schedule()
 end
 
 function BackgroundRunner:_insert(job)
-    assert(self ~= nil)
     job.insert_time = UIManager:getTime()
     table.insert(self.jobs, job)
 end
 
 BackgroundRunner:_schedule()
 
-local BackgroundRunnerWidget = WidgetContainer:new{
+local BackgroundRunnerWidget = WidgetContainer:extend{
     name = "backgroundrunner",
     runner = BackgroundRunner,
 }