Make it possible to select a Custom Segment using only keyboard #9616

Goal of this issue is to improve accessibility in Piwik. In particular, it should be possible, with keyboard only (no mouse), to select a custom segment to apply in the report.

Expected: with keyboard, one can open the Segment editor which lists the segments, and navigate through the segments using the arrow key (or TAB?), and select a custom segment to load (on ENTER).

Great work @CheweyZ - Thanks for your help getting this resolved!