Disable the delete button when selected profile is undeletable.

This is to avoid the confusion users might have: "Why can't I delete
the 'Shell' profile when the UI implies I can ?". Users tend to think
konsole is broken in that situation.

Current implementation follows a stupid logic:
1). When multiple profiles are selecte, always enable the delete button.
2). When single profile is selected, disable it if user has no
permission to delete that profile.