buffer: Use flexbox for status buffer.
buffer: Use flexbox for status buffer.

This uses flexbox for dynamic status content layout, and uses ems for the length
values. Tested on the screens starting from some 400px width (cropped controls
and modes, partially visible URL and <3 visible tabs) to the FullHD screen (all
visible, lots of space for modes and tabs, URL displaying in full, if necessary,
constant controls size.

Closes #2436.

jmercouris/NeXT,jmercouris/NeXT