[Notifications] Don't steal mouse clicks from TextEdit in SelectableLabel
Ensures built-in functionality like double- and triple-clicking text works.
Also lets us get rid of the custom text selection handling, replacing it with an even filter class.
BUG: 431398 FIXED-IN: 5.22.0
Won't do anything for notifications with default actions, so not sure how useful that is... perhaps for KDE Connect as mentioned in the bug report whose notifications are usually not clickable