feat: lockscreen screen filter (#2740)

this makes use of the existing getFilteredScreens function to allow
users to specify which displays the lockscreen should appear on. prior
to this change, it was "all" or just one.

quickshell/Common/settings/SettingsSpec.js

@@ -451,7 +451,6 @@ var SPEC = {
     maxFprintTries: { def: 15 },
     enableU2f: { def: false, onChange: "scheduleAuthApply" },
     u2fMode: { def: "or" },
-    lockScreenActiveMonitor: { def: "all" },
     lockScreenInactiveColor: { def: "#000000" },
     lockScreenNotificationMode: { def: 0 },
     lockScreenVideoEnabled: { def: false },