Add FFmpeg

[sakertooth]

Adds the FFmpeg library. Meant to replace sndfile in the future. Using FFmpeg comes with its own advantages, such as the support for a plethora of audio codecs and formats.

[JohannesLorenz]

As the build shows, your PR fails the CI on all machines:

CMake Error at /usr/share/cmake-3.10/Modules/FindPackageHandleStandardArgs.cmake:211 (message):
  No REQUIRED_VARS specified for FIND_PACKAGE_HANDLE_STANDARD_ARGS()
Call Stack (most recent call first):
  cmake/modules/FindFFmpeg.cmake:97 (find_package_handle_standard_args)
  CMakeLists.txt:199 (FIND_PACKAGE)

I recommend making this PR a draft until you get it working.

[LmmsBot]

:robot: Hey, I'm @LmmsBot from github.com/lmms/bot and I made downloads for this pull request, click me to make them magically appear! :tophat:

Linux

• Linux (AppImage): lmms-1.3.0-alpha.1.186+g9dbe66fa1-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.3.0-alpha.1.186+g9dbe66fa1-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.3.0-alpha.1.186+g9dbe66fa1-mingw-win64.exe (build link)

:robot:

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[sakertooth]

* Installing ffmpeg for MinGW should probably be moved to [`lmms-ci-docker`](https://github.com/LMMS/lmms-ci-docker) repository. I'll also ask @tobydox to create MinGW PPA packages for ffmpeg later.

Yeah, I figured that would be the best way to go about it but I realized tobydox didn't have FFmpeg packages in his PPA. To my surprise I thought I could get pre-built FFmpeg packages for MinGW somewhere else, but I suppose not.

[sakertooth]

Forgot to explain why I made this a draft over here.

I made this a draft because I'm waiting for @tobydox to add FFmpeg MinGW packages in his PPA. That way, we can move FFmpeg for MinGW to the docker images themselves without having to cross compile FFmpeg, as to which I'll adjust this PR accordingly. The "Install FFMpeg" step for Linux will also be moved to the docker images.