Improves skill selection

When selecting skills, the  rows in the skill table now have a tab
index, allowing them to be in focus. This in turn allows the capture of
the arrow-keys up and down.

In effect, you can now tab through the skills, adding or removing ranks
with the up- and down-arrows.

It also improves on the layout, making the ranks look like the sum in
style, and spacing the up/down chevrons from the ranks.
Hide skill chevrons when not in focus

This hides the skill chevrons, when the skill isn’t hovered over or in
focus.
Refactor skill javascript