All UI elements are accessible through the obs frontend api via
obs_frontend_api_get_main_window, except for the tray icon and its
elements. This commit adds an obs_frontend_api_get_system_tray function
which returns the pointer to the QSystemTrayIcon cast to void (much like
the QMainWindow pointer for the main window).

In commit d17ee208, we attempted to fix a race condition crash in the
Logitech plugin by deferring the "stream/recording/replay buffer active"
calls to the UI thread.  However, the Logitech plugin loop_function
funciton can call obs_frontend_streaming_active/etc functions while the
UI thread waits for the loop_function thread for many OBS events,
causing a hard lock in the Logitech plugin.  This fixes that by making
the obs_frontend_streaming_active/etc functions completely atomic
instead.  It's a bit of a hack but it's better than accessing objects.

Additionally, check for null output handlers.

Uses WaitConnection() when calling obs_frontend_set_current_scene to
ensure that the calling thread and the UI thread are synchronized when
setting the current scene via this function.

Closes obsproject/obs-studio#1234

Allows the ability to add a new scene collection via the frontend API.
Blocks until the scene collection has been successfully added to ensure
synchronization between the calling thread and the UI thread.

(Jim: Added detailed description to commit message)

Closes obsproject/obs-studio#1232

Allows the ability to perform multiple actions that would normally save,
and defer the save until all of those actions are complete.

Closes obsproject/obs-studio#1231

Closes jp9000/obs-studio#1106

Allows safely removing callbacks from within the callbacks themselves.

Allows loading data before actually loading any scene data.

With these new functions, plugin developers can enable, disable and
get the status of Studio Mode, as well as get and set the current
preview scene and transition the current preview to Program

Here is a list of the Studio Mode events:
   - Studio Mode enabled
   - Studio Mode disabled
   - Previewed scene in Studio Mode changed

This commit adds functions to get, set and save the service and its
settings, and is plugged straight into the existing internal functions
serving this purpose

(Jim: Fixed commit message and missing newline at end of file)

Closes jp9000/obs-studio#895

Replay buffer and recording should be separate in case the user wants to
start recording from a specific point rather being forced to reconfigure
for regular recording.

Creates a new button on the main window below the recording button for
turning on/off the replay buffer.

Was using main->outputHandler->StreamingActive() for the
obs_frontend_recording_active function.

Allows manipulating and modifying the front-end via plugins.