Marked lang insertion point.

Change-Id: I5145eb987c54b3eb07c8c93ae13bb204da7f45a1
Work in progress: language marker button in doc area

Looks more or less like the mockup at
https://trello-attachments.s3.amazonaws.com/52e98a603e6d08a53861025b/5363dc861f242dbd5eb6af22/4881x3543/879a1efeee2a2eb7ad4fa19e4de513e0/Apps_Language%26EditHistory_Sprint31.png
though not 100% exact just yet.

Questions:

* Should this be added to ToC as well?
* Do we need/want a single-character icon in the icon font for the Aあ bit?
  Or should we use the existing one that's different from the mockup?
* How should the button respond visually while being pressed?
* How should the marker be displayed when no languages available? Or leave it out as now?
* Exact wrapping/pixel-level details and localization questions...

Change-Id: I46f40a347ee5d0c60b9c9b320c4bffc8da780d07