Add find_icon_name helper

Find the first available name for
ThemedIcon icons.

Fallback to default name if cannot
be found.

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.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>