Add groups to the service dropdown

This is done with a separator, and is cleared and repopulated every time
the service window is re-opened (in case the user has changed the list
of groups since the window was closed).
Setup an extra panel for showing groups

This extra "service" panel will display the name of the group, its
playback mode, and whatever presets are in it.
Vertically align label to top

When displaying the information about the group, vertically align it to
the top of the stack widget.
Enable/disable preset save button

If the user selects a group, turn the preset save button off.
Additional information about changing groups

Adds this below the summary of the group.