Redefine visibility and priority bitmask type
Add ability to define priority and visibility with ShowNotificationX(..)
Change ShowNotificationX(...) default argument value to null string
Fix memory leak.

Screens for registered clients weren't deleted in MythUINotificationCenter destructor. Leak wouldn't only have existed when frontend exited
Obfuscate MythUINotificationCenter implementatin in a private class

Was getting annoying having to recompile everything after every single change
Fix infinite loop

If we have a registration from a client which will survive MythMainWindow, the screen would have been re-created, which would have re-created the screen stack, which would have caused an infinite loop in MythMainWindow destructor.
Only create a screen stack once.
Display loss of connection to backend as a notification instead of a popup.

The popup would re-appear as soon as dismissed, now the notification will only appear if it's been already showing for more than 5s

Fixes #11651
Continue try to reconnect with the backend, so the notification about the backend being available is shown quickly

The logic seems a bit convoluted, but this is to keep the same logic as introduced in 943b142dfe3c208e1fa83e74d4e6ec6ea262a49c