applet: return icon name from get_icon() whenever possible (bgo #740574)

Get rid of all unnecessary pixbuf passing and prepare for appindicator
support that requires icon name passing instead.

https://bugzilla.gnome.org/show_bug.cgi?id=740574
(cherry picked from commit 411415fe7aa6bc03e5326b606086dbe7e3f8d840)