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/filemanager/filemanagerfilesearcher.lua

@@ -4,23 +4,23 @@ local CenterContainer = require("ui/widget/container/centercontainer")
 local DocumentRegistry = require("document/documentregistry")
 local FileChooser = require("ui/widget/filechooser")
 local InfoMessage = require("ui/widget/infomessage")
-local InputContainer = require("ui/widget/container/inputcontainer")
 local InputDialog = require("ui/widget/inputdialog")
 local Menu = require("ui/widget/menu")
 local Size = require("ui/size")
 local UIManager = require("ui/uimanager")
+local WidgetContainer = require("ui/widget/container/widgetcontainer")
+local BaseUtil = require("ffi/util")
 local Utf8Proc = require("ffi/utf8proc")
 local lfs = require("libs/libkoreader-lfs")
-local BaseUtil = require("ffi/util")
 local util = require("util")
 local _ = require("gettext")
 local Screen = require("device").screen
 local T = require("ffi/util").template
 
-local FileSearcher = InputContainer:new{
-    dirs = {},
-    files = {},
-    results = {},
+local FileSearcher = WidgetContainer:extend{
+    dirs = nil, -- table
+    files = nil, -- table
+    results = nil, -- table
 
     case_sensitive = false,
     include_subfolders = true,
@@ -32,6 +32,12 @@ local sys_folders = { -- do not search in sys_folders
     ["/sys"] = true,
 }
 
+function FileSearcher:init()
+    self.dirs = {}
+    self.files = {}
+    self.results = {}
+end
+
 function FileSearcher:readDir()
     local ReaderUI = require("apps/reader/readerui")
     local show_unsupported = G_reader_settings:isTrue("show_unsupported")