[Alt-Tab] Remove Ctrl+a shortcut for activating a window without exiting Alt-Tab.

It doesn't work well with differing icon widths and dynamically created arrows.
[Alt-Tab] Create arrows on demand.
[Alt-Tab] Avoid slightly varying switcher-bar height on vertical scrolling.
[Alt-Tab] Restrict the display of the focus-indicator arrow.
[Alt-Tab] Fix a recent regression to fast Alt-Tab switching.

The short delay before showing the switcher had stopped working, showing
it instantly at all times.
[Alt-Tab] Move the arrow slightly up from the bottom of the switcher bar.