Improve terminology for control type

[westonruter]

Currently the Control::$type is designed to indicate whether it is a selector for a post, term, or user. However, there could very well be controls that are for editing these objects that would have these same types. I suggest that the type be fixed to something like object_multi_select and then to introduce a new class property like $selection_type to hold whether it is for a post, term, or user. And then another $selection_object to contain the kind of post or user that can be selected. (These variable names for type and object are derived from terminology used in nav menu items. Something better could be chosen.)

[westonruter]

Done in xwp/wp-customize-object-selector#1

[westonruter]

(Well, somewhat. No support for terms or users has been added yet.)