Vector: Componentize screen.less

This is a bold but much needed move.
I've not changed any of the css, I've simply separated it into separate
LESS files. This arguably makes the code much more readable and organised.
This is step 1 of a big cleanup of the Vector skin. A +2 is much appreciated.

Change-Id: I22e100bb0f9566af6ddccd88bc310f8902c5ff70
Merge "Vector: Componentize screen.less"