libobs: Add the ability to make sources obsolete

This allows the ability to do things such as change defaults or
properties of sources without necessarily breaking older user
configurations that had older defaults or properties.

docs/sphinx/reference-sources.rst

@@ -142,6 +142,17 @@ Source Definition Structure (obs_source_info)
      from creating an audio feedback loop.  This is primarily only used
      with desktop audio capture sources.
 
+   - **OBS_SOURCE_CAP_DISABLED** - This source type has been disabled
+     and should not be shown as a type of source the user can add.
+
+   - **OBS_SOURCE_CAP_OBSOLETE** - This source type is obsolete and
+     should not be shown as a type of source the user can add.
+     Identical to *OBS_SOURCE_CAP_DISABLED*.  Meant to be used when a
+     source has changed in some way (mostly defaults/properties), but
+     you want to avoid breaking older configurations.  Basically solves
+     the problem of "I want to change the defaults of a source but I
+     don't want to break people's configurations"
+
 .. member:: const char *(*obs_source_info.get_name)(void *type_data)
 
    Get the translated name of the source type.