The was_removed variable was rather unnecessary

Scene items previously were removed by calling obs_sceneitem_destroy,
but this proved to be a potential race condition where two different
threads could try to destroy the same scene item at the same time.

Instead of doing that, reference counting is now used on scene items,
and an explicit obs_sceneitem_remove function is used instead for item
removal, which sets a 'removed' variable to ensure it can only be called
exactly one time.

The previous commit used the scene as a  parameter to check to see if
the scene item was still present within the scene before destroying, but
this was actually unnecessary because the fault was because the destroy
signal was being triggered *before* the scene's mutex locked, thus
causing a race condition.  I changed the code so that it signals after
the lock instead of before, so the scene parameter should no longer be
necessary.

Prevents a double free in case the scene item was previously removed

Fixes a deadlock when trying to remove a source from the GUI. The scene
item signal handlers would mark the source as removed which results in
the video thread also trying to run obs_sceneitem_destroy thereby
deadlocking the video thread (and the GUI thread)

- Add 'set_default' functions to obs-data.*.  These functions ensure
  that a paramter exists and that the parameter is of a specific type.
  If not, it will create or overwrite the value with the default setting
  instead.

  These functions are meant to be explicitly called before using any of
  the 'get' functions.  The reason why it was designed this way is to
  encourage defaults to be set in a single place/function.

  For example, ideal usage is to create one function for your data,
  "set_my_defaults(obs_data_t data)", set all the default values within
  that function, and then call that function on create/update, that way
  all defaults are centralized to a single place.

- Ensure that data passed to sources/encoders/outputs/etc is always
  valid, and not a null value.

- While I'm remembering, fix a few defaults of the main program config
  file data.

Add a fairly easy to use settings interface that can be passed to
plugins, and replaced the old character string system that was being
used before.  The new data interface allows for an easier method of
getting/altering settings for plugins, and is built to be serializable
to/from JSON.

Also, removed another wxFormBuilder file that was no longer in use.

  Scenes will now signal via their source when an item has been added
or removed from them.

  "add" - Item added to the scene.
  Parameters:  "scene": Scene that the item was added to.
               "item":  Item that was added.

  "remove" - Item removed from the scene.
  Parameters:  "scene": Scene that the item was removed from.
               "item":  Item that was removed.

make scene items use linked list rather than array (containers are a bad habit), add a few functions for getting scene item information, also use a mutex to prevent race conditions when using the linked list

With the permission of my fellow contributors, I'm switching obs-studio back to GPL v2+ to prevent issues between this project and the original OBS project, and for personal reasons to avoid legal ambiguity (not political reasons, I admittedly would prefer GPL v3+)

cleaned up main internal data structure design, changed to reference counting for sources to ensure safe destruction of source objects from all parts of the system, added some service-related stuff for testing

added support for jansson, added new output files, made some adjustments to the API, fixed a UI subclass issue

added intial async audio/video code, fixed a few bugs, improved thread safety, and made a few other minor adjustments

change names, fix some bugs, minor GL/D3D fixes, update tests, fix effect files, output a little more debug information