Information Overlay

Addresses Issue https://github.com/popcornmix/omxplayer/issues/9.

### Overview

* Adds method: OMXPlayerSubtitles::ShowInfo(string info, int duration = 2000)
* A new argv flag (--no-info-overlay) disables showing the information.

Calling ShowInfo stops the subtitle rendering, displays the specified info string, and waits duration milliseconds before showing subtitles again.  If the current media has no subtitles, the ShowInfo method is the only method in the OMXPlayerSubtitles class that is allowed to be called.

Currently provides information messages when these are changed:
* Play/Pause
* Volume
* Seek / Chapter 
* Subtitle delay
* Audio Channel

### Symbols

Unicode characters are being used for the play and pause symbols.

### Issues

Any info displayed while the media is paused will not get removed from the screen.  i.e., if you press pause, then change the volume, the 'Volume: ' overlay will stay on the screen until play resumes.  This is because the OMXPlayerSubtitles class uses the media clock to determine when to change subtitles.