Add a setting to keep normal notifications always on top
In discussion there Nate said that it's fine to reuse
PlasmaCore.Dialog.CriticalNotification type for normal notifications, if user wants them to show up on top.
I thought that such change wouldn't really be convenient for most users, as notifications over fullscreen windows wouldn't time out (because they would be considered critical), so I didn't try to make this into a proper patch. But while using it I found out that such notifications do actually time out. I guess that the thing that is responsible for timing them out looks at the urgency, not the type, which is really convenient. So I went ahead and tried to make this into a proper patch that hopefully can be accepted in upstream.
There also will be a pretty straightforward companion MR to
plasma-desktop, to add this setting into UI.