[ENH] Display Widgets on Top option

[astaric]

Issue

When Canvas gets focus, all widgets get hidden behind it

Description of changes

View -> Float Widgets on Top flag to show widgets on top of all other windows.

[astaric]

@ales-erjavec, can you take a look?

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[ales-erjavec]

On macOS all windows with the Qt.WindowStaysOnTop hint stack above other windows including windows belonging to other applications.
Perhaps we can use QGuiApplication.applicationStateChanged to unset/set the flag around the state changes.

[ales-erjavec]

Change this to a setter (def set_show_widgets_on_top(self, state):) record the state in an instance attribute, and use that instead of querying QSettings in __set_float_on_top, so the state is local to the workflow and not shared between them.

[astaric]

@ales-erjavec , state is now stored as an instance attribute in WidgetManager.

[lanzagar]

Does not really achieve the goal completely on my system (using dwm :)), but I could get the desired functionality even before this PR (and can still).
It does bring all widgets to the top when selected so it might be useful for that (if we add a shortcut).