Stay-on-top prevents floating windows to get focus

Related to #6115

When "stay on top" on an instrument window is used it will create artefacts on larger windows and also prevent floating windows to gain focus

#### Steps to reproduce
**Bug with floating windows**
In order to have several windows for testing, open an example project.
* Pick any instrument and place it anywhere over song-editor.
* Use **stay on top** for the instrument
* Open Piano-roll as a floating window
We now have this:
Click anywhere on the piano-roll -f.i. in the yellow marking.
Piano-roll 'disappear' or is rather sent to the back of the windows, including song-editor, so it seams to disappear.
If **Stay on top** is disabled on the instrument, it is again possible to work in piano-roll
**_Random_ occurring Bug with _non_-floating windows**
_This happens unpredictable and is not reproduceable_
If Stay on top** is activated on an instrument it is not possible to open any other full-screen window on _top_ of the stack.
Clicking the toggle-button or using hot-key for any window will open that window on the bottom of the stack and below song-editor

#### Expected behavior
Stay on top** instrument should overlap piano-roll

#### Actual behavior
Stay on top** instrument should not have the result that piano-roll window is sent to the bottom of the stack of floating windows, if it is clicked

#### Affected LMMS versions
windows 1.21, 1.22 & 1.3 Alpha

It is strange that this has not been reported before. It makes me believe that no one is using **Stay on top**

Reproduce the issue on Windows with an example project by placing an instrument over the song editor, enabling Stay on top, and opening the Piano-roll as a floating window. Trace the Stay on top and floating-window focus behavior; done means the instrument remains above the Piano-roll without sending it or other windows to the bottom of the stack.

Written by the indexing model from the issue text.