Add RTL support to the editor

[maxme]

Also, check how it works with the legacy editor.

[rachelmcr]

The legacy editor detects RTL languages (e.g. when the system or keyboard language is RTL) and auto-aligns to the right when you start typing, in both the post title and body fields:

The visual editor, on the other hand, keeps the left alignment even when the system/keyboard language is switched to RTL:

Tested on Nexus 9, Android 6.0.1, WPAndroid alpha-16