UX improvements for "On Top" menu option

[tmiw]

Resolves #546 by doing the following:

1. On Linux (or any other platform using GTK), hide the "On Top" menu option as it's not reliable (note: support was even removed entirely in GTK4 for this reason).
2. For other platforms, rename "On Top" to "Keep On Top" and indicate whether it's enabled via a checkmark next to the menu item. Example below:

[barjac]

I think the common term for this is 'Keep above others' as in KDE Plasma5.

[tmiw]

Windows calls it "Always on Top", IIRC. I don't know if macOS uses the same or different terminology for this. We'd probably want to use one of those if we were to use something other than "Keep On Top".