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/device/generic/powerd.lua

@@ -73,12 +73,10 @@ function BasePowerD:beforeSuspend() end
 function BasePowerD:afterResume() end
 
 function BasePowerD:isFrontlightOn()
-    assert(self ~= nil)
     return self.is_fl_on
 end
 
 function BasePowerD:_decideFrontlightState()
-    assert(self ~= nil)
     assert(self.device:hasFrontlight())
     self.is_fl_on = self:isFrontlightOnHW()
 end
@@ -88,7 +86,6 @@ function BasePowerD:isFrontlightOff()
 end
 
 function BasePowerD:frontlightIntensity()
-    assert(self ~= nil)
     if not self.device:hasFrontlight() then return 0 end
     if self:isFrontlightOff() then return 0 end
     --- @note: We assume that nothing other than us will set the frontlight level,
@@ -98,7 +95,6 @@ function BasePowerD:frontlightIntensity()
 end
 
 function BasePowerD:toggleFrontlight()
-    assert(self ~= nil)
     if not self.device:hasFrontlight() then return false end
     if self:isFrontlightOn() then
         return self:turnOffFrontlight()
@@ -108,7 +104,6 @@ function BasePowerD:toggleFrontlight()
 end
 
 function BasePowerD:turnOffFrontlight()
-    assert(self ~= nil)
     if not self.device:hasFrontlight() then return end
     if self:isFrontlightOff() then return false end
     self:turnOffFrontlightHW()
@@ -118,7 +113,6 @@ function BasePowerD:turnOffFrontlight()
 end
 
 function BasePowerD:turnOnFrontlight()
-    assert(self ~= nil)
     if not self.device:hasFrontlight() then return end
     if self:isFrontlightOn() then return false end
     if self.fl_intensity == self.fl_min then return false end  --- @fixme what the hell?
@@ -129,7 +123,6 @@ function BasePowerD:turnOnFrontlight()
 end
 
 function BasePowerD:frontlightWarmth()
-    assert(self ~= nil)
     if not self.device:hasNaturalLight() then
         return 0
     end