window: Tweak visible focus behavior

window: Tweak visible focus behavior

Only turn on visible focus when a key event actually leads
to a change in focus location (ie, 'keynav').

Make the visible focus disappear after 5 seconds of no
keyboard interaction, to avoid permanent focus ring
distraction.

As an extra bonus, make it so that we make the focus
visible while the Alt key is pressed. This gives us
a 'find my focus!' shortcut, and goes well with the
prexisting use of Alt for finding mnemonics.

Discussed in: #2644