Add include for ptrdiff_t
Change semantics of obs-data default values

Default values are now permanently stored in the obs_data_items and
can be accessed via the new get_default functions

Also default values are no longer serialized to JSON to ease transition
to new default values
Add obs-data autoselect functions

These are meant to reflect auto-detection configuration changes that
should not be written to the config, for example, frame rate changes
for a camera where the (user-/config-file-)configured frame rate isn't
available but a similar frame rate can be automatically chosen
Make properties-view reflect autoselect settings (for property lists)