Plugin-based panel group configuration

Panels can be added through plugin configuration, but it is broken
if the user can only use existing panel groups. This adds the
capability to create an empty panel group that can be used by panel
configuration to add itself to the group.

Change-Id: I28d6072fc05b3a4ef5a41cd68337a7864c560525
Closes-Bug: #1287981
Merge "Plugin-based panel group configuration"