BUG: confirm delete of markups

Add a message box to tell the user how many markups they
are about to delete and let them cancel. Option to
not show the message again.

COMP: cleaned up unused variable warnings

Fixes issue #3324



git-svn-id: http://svn.slicer.org/Slicer4/trunk@22332 3bd1e089-480b-0410-8dfb-8563597acbee