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/textboxwidget.lua

@@ -33,7 +33,7 @@ local util = require("util")
 local xtext -- Delayed (and optional) loading
 local Screen = require("device").screen
 
-local TextBoxWidget = InputContainer:new{
+local TextBoxWidget = InputContainer:extend{
     text = nil,
     editable = false, -- Editable flag for whether drawing the cursor or not.
     justified = false, -- Should text be justified (spaces widened to fill width)
@@ -59,6 +59,7 @@ local TextBoxWidget = InputContainer:new{
     idx_pad = nil,     -- idx => pad for char at idx, if non zero
     vertical_string_list = nil,
     virtual_line_num = 1, -- index of the top displayed line
+    current_line_num = 1, -- index of the currently edited line (i.e., where our cursor is)
     line_height_px = nil, -- height of a line in px
     lines_per_page = nil, -- number of visible lines
     text_height = nil,    -- adjusted height to visible text (lines_per_page*line_height_px)
@@ -1107,8 +1108,8 @@ function TextBoxWidget:getFontSizeToFitHeight(height_px, nb_lines, line_height_e
 end
 
 function TextBoxWidget:getCharPos()
-    -- returns virtual_line_num too
-    return self.charpos, self.virtual_line_num
+    -- returns virtual_line_num & current_line_num too
+    return self.charpos, self.virtual_line_num, self.current_line_num
 end
 
 function TextBoxWidget:getSize()
@@ -1570,7 +1571,8 @@ local CURSOR_USE_REFRESH_FUNCS = G_reader_settings:nilOrTrue("ui_cursor_use_refr
 function TextBoxWidget:moveCursorToCharPos(charpos)
     self.charpos = charpos
     self.prev_virtual_line_num = self.virtual_line_num
-    local x, y = self:_getXYForCharPos() -- we can get y outside current view
+    local x, y, screen_line_num = self:_getXYForCharPos() -- we can get y outside current view
+    self.current_line_num = screen_line_num
     -- adjust self.virtual_line_num for overflowed y to have y in current view
     if y < 0 then
         local scroll_lines = math.ceil( -y / self.line_height_px )
@@ -1811,6 +1813,14 @@ function TextBoxWidget:moveCursorDown()
     self:moveCursorToXY(x, y + self.line_height_px)
 end
 
+function TextBoxWidget:moveCursorHome()
+    self:moveCursorToCharPos(self.vertical_string_list[self.current_line_num].offset)
+end
+
+function TextBoxWidget:moveCursorEnd()
+    self:moveCursorToCharPos(self.vertical_string_list[self.current_line_num+1] and self.vertical_string_list[self.current_line_num+1].offset - 1 or #self.charlist + 1)
+end
+
 
 --- Text selection with Hold