Feature request: Add visual feedback of the just-focused window

[markstos]

When focusing a borderless window, there may be no immediate feedback to confirm which window has been focused.

After #82, an update was made which makes the hint on the current window look different by inverting the colors.

The proposal here builds on the idea of hinting the current window by visually showing the focus change windows:

After focus switches to the new window, the hints remain for a very short period (probably even < 1 second would do). During this time the color of the hint would change to match the "current window" color scheme to give visual confirmation where the focus has gone. At the same time, coloring of the window switched from loses it's "current window" decoration scheme.

Since this confirmation is happening after the new window has focused, the new window remains immediately ready for input without waiting for the confirmation visualization to complete.