Adds API:
obs_scene_add_group2
obs_scene_insert_group2
obs_sceneitem_group_ungroup2

These functions should be used by plugins if they need to use these
functions and need to send a refresh signal.  If a major API rework ever
happens the old functions should be removed.  The old functions should
eventually be deprecated.

The reason why specifying a 'signal' parameter is useful is because it's
a bit more seamless for the user interface to be able to have custom
handling of these specific cases.  It looks better and doesn't require
completely erasing/recreating the entire list, which is visually
unappealing.

Fixes a bug where the UI would not know that a group had been updated.

This signal is used to specify when a scene needs a full refresh of its
item list.

docs/sphinx: add media controls and source icon

Fix OpenGL thread-safety on Mac to use latest Xcode

README.rst: Remove retired CI services

UI: Use absolute path for portable mode multi-instance check

UI: Show source icons in Advanced Audio Properties

Travis and AppVeyor have been retired in favor of Azure Pipelines.

Also re-order Crowdin and Discord badges and fix indentation of Azure image.

UI: Fix transitions being disabled

win-capture: Faster display / window capture updates

UI: Always prompt when updates are available

obs-ffmpeg, vlc-video: Add media control support

Co-authored-by: NVodBox <dillon@vodbox.io>

When changing the target for display or window capture, force a
refresh of the source by setting the timer to fire immediately. This
removes 1-3 seconds of "lag" before the new display or window is
visible and makes the UI feel more responsive.

Closes https://github.com/obsproject/obs-studio/issues/2322

The current updater code won't prompt the user if an output is active or
if game capture is in use. These are both fairly common situations to be
in as soon as OBS starts up - for example, the NDI and VirtualCam
plugins both start outputs automatically, and a game capture source will
hook if the game is open.

It should be fairly obvious to a user that OBS will be closed when an
update is installed. Given the low frequency of game capture updates,
the risk of an update encountering in-use files is also very low, and
the standalone updater UI already shows a prompt should this happen.

obs-outputs: Various librtmp mbedtls fixes

Previously the assertion required the idx to be smaller than the number
of elements in the darray. This would mean you could not insert anything
at the end of a darray, and would make it impossible to insert an array
into an empty darray.

This param is the buffer size, including null terminator

The last parameter is the buffer size (which would include null
terminator), not the desired string length.

obs-transitions: Halve stinger padding to 250ms

Triggering a new transition too soon after a
stinger would cause the wrong scene to be displayed

libobs: Add media control support to backend

media-playback: Add functions to pause/seek media source

UI: Use input validator on resolution line edit

UI: Fix transtions not enabled

The report in https://github.com/obsproject/obs-studio/issues/2350
identified the issue as being caused by mbedtls not following symbolic
links, but it turns out the issue was the mbedtls_x509_crt_parse_path
return value which was already fixed in 4d89123c. So these changes are
no longer necessary.