Remove editor switching infrastructure in favor of VE's source mode

Authored by Catrope on Oct 21 2017, 5:28 AM.

Description

Remove editor switching infrastructure in favor of VE's source mode

This simplifies a lot of code. Essentially, EditorSwitcherWidget,
VisualEditorWidget and bits of WikitextEditorWidget are all merged into
EditorWidget.

Also adds a textarea fallback when VE is not available or supported,
and adds a preference that allows users to disable VE and get the
textarea fallback instead.

Bug: T155861
Change-Id: Ie8014a64eae5f1b24495d6108e720464a8e75d9c

Details