libobs: Add property groups

Property groups allow multiple properties to be combined into a single
visual group and thus can be used to reduce visual clutter and improve
user experience by giving additional context to a property. The name of
a child of a group is not affected by the group and thus all property
names must still be unique.

A new parent field has been added to obs_properties_t to ease the
tracking of existing properties. That way properties inside a group will
also be able to verify that they have a unique name.
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