Rework panel icon handling

This should fix the panel icon issue introduced in a2a12efcf8c263e5615155ef2d47bb2aff796f88 (issue #33) properly and finally resolve #23. The "fix" suggested in 66b750c5c63c78da6700e542f432776603409fd3 wasn't sufficient as it still used the audio-volume-*-panel icon for OSD notifications which should always use the system's GTK icon theme instead. Moreover, this PR adds an option to the preferences window to choose whether volumeicon should try and use the *-panel icon variants or not.