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/luasettings.lua

@@ -9,16 +9,19 @@ local logger = require("logger")
 
 local LuaSettings = {}
 
-function LuaSettings:new(o)
+function LuaSettings:extend(o)
     o = o or {}
     setmetatable(o, self)
     self.__index = self
     return o
 end
+-- NOTE: Instances are created via open, so we do *NOT* implement a new method, to avoid confusion.
 
 --- Opens a settings file.
 function LuaSettings:open(file_path)
-    local new = {file=file_path}
+    local new = LuaSettings:extend{
+        file = file_path,
+    }
     local ok, stored
 
     -- File being absent and returning an empty table is a use case,
@@ -41,13 +44,13 @@ function LuaSettings:open(file_path)
         end
     end
 
-    return setmetatable(new, {__index = LuaSettings})
+    return new
 end
 
---- @todo DocSettings can return a LuaSettings to use following awesome features.
 function LuaSettings:wrap(data)
-    local new = {data = type(data) == "table" and data or {}}
-    return setmetatable(new, {__index = LuaSettings})
+    return self:extend{
+        data = type(data) == "table" and data or {},
+    }
 end
 
 --[[--Reads child settings.
@@ -66,7 +69,7 @@ end
     -- result "b"
 ]]
 function LuaSettings:child(key)
-    return LuaSettings:wrap(self:readSetting(key))
+    return self:wrap(self:readSetting(key))
 end
 
 --[[-- Reads a setting, optionally initializing it to a default.
@@ -251,11 +254,10 @@ function LuaSettings:flush()
     if not self.file then return end
     local directory_updated = false
     if lfs.attributes(self.file, "mode") == "file" then
-        -- As an additional safety measure (to the ffiutil.fsync* calls
-        -- used below), we only backup the file to .old when it has
-        -- not been modified in the last 60 seconds. This should ensure
-        -- in the case the fsync calls are not supported that the OS
-        -- may have itself sync'ed that file content in the meantime.
+        -- As an additional safety measure (to the ffiutil.fsync* calls used below),
+        -- we only backup the file to .old when it has not been modified in the last 60 seconds.
+        -- This should ensure in the case the fsync calls are not supported
+        -- that the OS may have itself sync'ed that file content in the meantime.
         local mtime = lfs.attributes(self.file, "modification")
         if mtime < os.time() - 60 then
             os.rename(self.file, self.file .. ".old")