Use find_icon_name method

Use the find_icon_name method instead of
repeating this search for volumes icons
names all over.

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>

This change depends on https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/26