Add 'always on top' support across OSX spaces

[zanona]

Hey @yuhua-chen, thanks a lot for LayerX, it's great 👏.

I have one feature request if you don't mind:

It would be great if images stood always on top even when switching between different spaces. Currently switching to a different space will leave LayerX window where it was, not following you through.

[krin-san]

Already exists. Just toggle both Window -> Keep on top and Window -> Keep on all spaces