UI: Add function to find default video save path

On windows this will return the documents\video directory, but on
linux/mac it'll just return $HOME for the time being because I don't
know if there really are any other appropriate adequate paths to use.
Perhaps someone else can be willing to fill this in if they wish.
UI: Add function to generate time/date filenames
UI: Add FLV file output (sharing encoders)

Implement the 'file path' in output settings, and implement the 'start
recording' button, though for the time being I'm just going to make it
use a directory rather than allow custom file names.

This file output will actually share the video and audio encoder with
the stream.

I don't really know what to do about MP4 -- I don't really like the idea
of saving directly in the program, if you do and the program crashes,
that MP4 file is lost.  I'm contemplating making some sort of mp4 output
process stub.  So no MP4 file output for the time being.

If you need MP4, just remux it with FFmpeg:

ffmpeg -i flv_file.flv -acodec copy -vcodec copy mp4_file.mp4