[Notifications] Use explicit launcher job instead of URL invocation
Allows us to run executables, e.g. downloaded installers. Also moves some of the logic to the C++ side.
It also indicates "openwith" Kiosk restriction in the UI now by hiding the button.
- "Open with Gwenview" still shows icon and opens file in Gwenview
- "Open with..." when having downloaded a file without associated application shows open with dialog
- "Open with..." disappears and "More options" becoming a labeled button when
openwith
is restricted through Kiosk - "Open containing folder" still shows and works when downloading multiple files
- Generic "Open" while mime hasn't been determined yet does a URL invocation as before
- Notification popup hides when invoked as before
Seems Qt.openUrlExternally
changed behavior at some point...