Don't allow virtual keyboard window to receive keyboard events

It seems that depending on the OS/Window Manager being used, the virtual keyboard (as well as other floating windows) sometimes gain focus and steal all keyboard-related events. This is an interesting issue because simply moving the mouse over a window in one window manager might focus it while even clicking doesn't permanently focus the same window in a different window manager.

In my case, the virtual keyboard gains focus on mouse over, stealing keyboard events such as the numbers for note durations, backspace for undo, etc. My window manager is BSPWM.

It is possible that this is an issue with the window manager and not with musescore, but I doubt that it is isolated to bspwm, and thus it seems to me that it would be a good idea to have a workaround on the musescore end in case there is another WM out there with the same issue.

See https://musescore.org/en/node/317926