config.{c,h}: add option for panel icons

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Use *-panel icons for the statusicon if requested

This adds proper support for the audio-volume-*-panel icon variants. If
requested in the config, we now try to use audio-volume-*-panel for the
statusicon if the default GTK icon theme is used. Otherwise we fall back
on the regular audio-volume-* variants. Note that the notification icons
always use the system's GTK icon theme again now. We mistakenly changed
this to use the *-panel versions as well at some point.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Preferences: add option to use *-panel icons

This adds the ability to change whether audio-volume-*-panel icon
variants should be used or not to the preferences window.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Preferences: change panel option label

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
preferences.ui: reset required GTK+ version to 3.0

Bumping the required GTK+ version in Glade in commit 90c29a9 was
unnecessary as volumeicon still only uses GTK+-3.0 widgets.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Merge pull request #39 from nkoep/rework-panel-icon-handling

Rework panel icon handling