Implement scene collections in menu only

This hopefully resolves some conflicting opinions about how scene
collections should be handled, and removes it from the general settings
panel, where it felt a bit cluttered.

Actually, I would kind of like it if profiles were handled this way too.
This feels a lot cleaner than how profiles are handled in the general
settings.  The profiles thing in the general settings definitely feels
clunky.

In the main menu, the scene collections menu now has the following menu
items:

- New
- Duplicate
- Rename
- Remove
- Import
- Export

Duplicate was something I personally added - I felt like it might be a
nice thing to have for the sake of convenience.  Import/Export was
requested by dodgepong, and is also definitely nice to have.  Due to how
easy new scene collections are to set up it didn't take very long to put
in.
Merge pull request #358 from jp9000/scene-collection-menu

Implement scene collections in menu only
Fix startup issues with scene collections

If a scene collection wasn't found at startup, it would create a
'(null)' scene due to a null pointer.

Also, moved scene collections to the scene collections folder rather
than the base folder to prevent conflicts with the older scenes.xconfig
file.
Copy scene collection to scenes.xconfig

This allows better backward compatibility with older versions.  This
will keep the current scene collection set to default if the user ever
decides to downgrade OBS versions for whatever reason.
Fix deadlock if encoder never received a picture
Prevent and warn about excessive audio buffering
Fix audio thread deadlock and issues with PTT / mute
Use FindWindow for DWM monitor capture

Because the DWM window is an invisible window, the recent change using
FindVisibleWindow instead of FindWindow will cause it to never find the
DWM window.  So, detect if the target is the DWM, and then use regular
FindWindow instead if so.
Merge pull request #2 from jp9000/master