[Alt-Tab] Various small refactorings.
[Alt-Tab] Indicate the window currently having focus with an upward-pointing arrow.
[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.