Add get mount icon name try2

Refactor code that searches the first available
icon name for given a mount point and icon size.
This search is limited to ThemedIcons.
   
Fallback to a default icon name if cannot be
found or not a ThemedIcon.
   
This patch solves the issue reported by
Suraj. Volumes with GFileIcon icons caused
Journal volumes toolbar, Journal copy-to
menu items and Frame device icon to break.