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/ui/widget/bookstatuswidget.lua

@@ -32,37 +32,32 @@ local T = require("ffi/util").template
 local stats_book = {}
 
 --[[
---Save into sdr folder addtional section
+-- Stored in the sidecar metadata, in a dedicated table:
 ["summary"] = {
     ["rating"] = 5,
     ["note"] = "Some text",
     ["status"] = "Reading"
     ["modified"] = "24.01.2016"
 },]]
-local BookStatusWidget = FocusManager:new{
+local BookStatusWidget = FocusManager:extend{
     padding = Size.padding.fullscreen,
     settings = nil,
     thumbnail = nil,
     props = nil,
-    star = {},
-    summary = {
-        rating = nil,
-        note = nil,
-        status = "",
-        modified = "",
-    },
+    star = nil, -- Button
+    summary = nil, -- hash
 }
 
 function BookStatusWidget:init()
     self.layout = {}
+    -- What a blank, full summary table should look like
+    local new_summary = {
+        rating = nil,
+        note = nil,
+        status = "",
+        modified = "",
+    }
     if self.settings then
-        -- What a blank, full summary table should look like
-        local new_summary = {
-            rating = nil,
-            note = nil,
-            status = "",
-            modified = "",
-        }
         local summary = self.settings:readSetting("summary")
         -- Check if the summary table we get is a full one, or a minimal one from CoverMenu...
         if summary then
@@ -77,6 +72,8 @@ function BookStatusWidget:init()
         else
             self.summary = new_summary
         end
+    else
+        self.summary = new_summary
     end
     self.total_pages = self.ui.document:getPageCount()
     stats_book = self:getStats()