CI: Build on Linux with AppVeyor

On Ubuntu 16.04 (Xenial), we have to build mbedTLS from source because
the version available from apt, mbedTLS 2.2.1, does not work with the
RTMPS implementation in OBS, which requires mbedTLS 2.7 at minimum. This
is due to the use of the function mbedtls_ssl_conf_dh_param_bin, which
was introduced in mbedTLS 2.7.

Build on Linux (Ubuntu 16.04) with AppVeyor. Disable PowerShell on
Linux. Ubuntu 16.04's dependencies are different from the existing
Ubuntu 14.04 dependencies, so a separate script for installing those
dependencies has been added.

AppVeyor commands use cmd/batch by default. Specify cmd for lines meant
for cmd to differentiate them from lines meant for ps/PowerShell or
sh/shell/Bash.

Move the AppVeyor install scripts to a separate script file. This
shortens the appveyor.yml file and makes it easier to introduce new
install scripts for other build environments.

There is one minor change to the install script, and that was to change
directory back to the obs-studio git root directory.

obs-filters: Add base canvas resolution option

UI: Fix array length computation

The UI currently is dependent on browser source files being present,
when in actuality the files may not have been fetched (i.e. not fetching
submodules).  This fixes that by using a macro to detect whether it's
available or not.

Closes obsproject/obs-studio#1490

docs/sphinx: Add missing obs_sceneitem_get_id info

win-capture: Add option to adjust hook rate for game capture

UI: Fix scrollbar alignment for themes Dark, Acri

Fixes an issue with horizontal scrollbar when it was hard to drag it by
mouse in Acri theme, because active area was thinner than the visible
scroll handle.

Fixes an issue when it was hard to drag by mouse the scroll handle in
Dark theme if scroll handle size is close to full size
(https://obsproject.com/mantis/view.php?id=466) or close to minimal
size.

Also, the mouse pointer was misaligned to the scroll handle when
moving: the scroll handle always moves faster than mouse pointer itself
and finishes first.

fix heap corruption when video capture thread release the frame before it push into source->async_frames.

obs-filters: Optimize chroma key's box filter

image-source: Decrease slideshow source memory usage

On KDE and Unity some QT events aren't triggered as explicit stated
by QT doc.  This is needed to at least show a fixed Context Menu.

Closes obsproject/obs-studio#1252

Plugins: Add descriptions to modules

Consolidate CFString conversions

The stats dock was incorrectly added to the .ui file, causing the stats
dock to show by default on startup, and just generally causing a bad
default startup UI appearance.  This fixes that by adding the stats dock
programmatically/dynamically instead of modifying the .ui file.

obs-qsv: Expose additional QSV encoder settings through GUI

Decklink cards accept 2, 8 or 16 audio channels.
If obs audio output is setup to n channels ( 8 >= n > 2), and the
decklink card captures n channels, one needs to squash the silent
channels (n+1, .., 8) or sampling issues occur.
This had been done on windows but is required also on macOs and linux.
The commit adds the fix for macOS and linux.
Some code factoring has also been done.

Closes obsproject/obs-studio#1350

Closes obsproject/obs-studio#1343

Add support for using the VLC source to play instant replays.

Closes obsproject/obs-studio#1334

This changes the remux dialog to support a collection of input/output
pairs, presented as a QTableView.  Standard Qt icons are used to
indicate the state of each entry during a remux operation.  Drag/drop
support is added to populate the list quickly.  Both Dark and Rachni
themes are updated to make QTableView look reasonable.

Relevant text is added in the localization files.

Closes obsproject/obs-studio#1153

UI: Change about dialog bottom color (light theme)