Update according to changes in libmatemixer
Improve GvcComboBox and related code

The combo box now includes an own reference to MateMixerSwitch. This simplifies GvcMixerDialog and makes the code more readable.

This change also includes a fix for oversized profile switch label and asynchronous updates of profile names in the HW tab.
Fix look of the peak meter on GTK3 and use a support function from libmate-desktop instead of custom copy&paste
Remove extraneous variable from configure.ac