Make floating move & resize work on the screenshot UI area

[YaLTeR]

Now that we have floating window movement, the same movement could naturally apply to the screenshot UI area.

Though, need to figure out how to best do it input-wise, since the screenshot UI intercepts the keys early. Should probably first check if it's a screenshot UI bind, then if not, run the usual search against configured binds, and if it matches one of the move/resize actions, apply it to the screenshot UI area.

niri msg action ones won't apply to the screenshot UI because they apply to the underlying layout even when the screenshot UI is up.

Idea by @FreeFull