Add MPlayer class

This is a slightly modified and updated version of the original MPlayer
from the player project.
MPlayer: add subtitles, volume and audio functions
MPlayer: add lengthChanged and positionChanged signals
MPlayer: tweak position/length timer
Add player and strap it in

Only Stop, Play and Pause buttons work for now.
Add Close menu items for Subtitles and Media
MPlayer: use pausingKeepForce in timeUpdate

Otherwise, the mplayer gets unpaused.
Relax update interval a bit.
Set filename after updateSubtitlesTable

Otherwise, updateSubtitlesTable calls clearSubtitlesTable, which clears
the filename.
Play currently open subtitles
Use libass for subtitle rendering

May need to make it an option later on.
Add MediaWidget class

This class automatically adjusts its height based on given aspect ratio.
Use MediaWidget and set media aspect ratio to 16:9
MediaWidget: allow 0 aspect ratio

And interpret it as "don't enforce aspect ratio".
Stop mplayer before opening new media file.
MPlayer: add set_ratio function

Add set_ prefix to osd, audio, subtitles, volume.
Singularize subtitles.
GUI bits for aspect ratio
Rename A1, B1, A2, B2 to A, B, C, D
Tweak ration buttons
Bootstrap ratio buttons
Fix 2.39 aspect ratio button