Add ‘MostlyOnTop’ mode for AlwaysOnTop

### Description of the new feature / enhancement

Instead of being always-on-top, the mostly-on-top window could be covered by the active window, i.e. by a window having the focus. However, as soon as the focus is moved to other window, the mostly-on-top window would be moved to the forward.

### Scenario when this would be used?

Mainly in the multi-screen setup if I want some window to be always on top on the secondary screen, however, occasionally I want to work shortly with some app on that screen the should temporarily overlap even the mostly-on-top window. When the focus is moved to another app, the mostly-on-top window pop-up to the foreground againto be visible over all other apps on the screen.

The specific example: Notebook screen with external monitor attached. I have a Zoom meeting set always-on-top on the small notebook screen working with apps primarily on the external monitor. However, I have a calculator and plain text editor for quick notes opened on the small notebook screen. The Zoom meeting window should be always visible as long as I am working on the external monitor. When switching to the calculator app, I should temporally overlap the Zoom window but the Zoom window should pop-up to the foreground immediately when activating another app on the external monitor making the Zoom meeting visible again with no need to rearrange windows on the notebook screen.

### Supporting information

_No response_

No files or tests are named. Start by locating the AlwaysOnTop implementation and its window-focus handling; define done as a mostly-on-top mode that can be covered by the focused window and returns to the foreground when focus moves elsewhere, including the described multi-screen scenario.

Written by the indexing model from the issue text.