Selections: Preserve selection/cursor position in translation

Adds event handlers for keyup and mouseup to capture selection
* Only captures selection inside translation container
* Uses mw.cx.selection to capture and save selection

Adds code to processPostMT to restore focus after MT
* Uses browser behavior to move cursor to beginning of section
* Captures and saves new cursor position

Change-Id: Ifa1fb9ca41109c5a94d2d4dcc0a0fdc85b924c74
Merge "Selections: Preserve selection/cursor position in translation"