MainForm.cs: full screen mode available to all media types
Consider this situation. The user is playing audio files, and some of the
album art is detected by Baka MPlayer but not all of them (e.g. m4a
files). mpv can detect album art within a file if it exists. mpv plays the
audio file with its detected album art as the video. When it does, Baka
MPlayer can't fully determine when mpv is truely playing an audio file or
not. So just allow audio files to go into full screen mode too.