Add UIName method to MSubtitle

- For representing a subtitle in the UI. We want
  something as descriptive as possible to
  differentiate unnamed subtitle tracks.
Use MSubtitle's new UIName instead of name

- Also remove some cases that would cause more
  than one subtitle to be enabled.
- Remove one case of a redundant "subtitle" text
  as it appears in the UIName..
Remove Enable/Disable alternate subtitle menuitems

- Since only one subtitle track can be enabled at a time
  this isn't needed.
Get subtitle selection rotation working again

- Now the "Subtitle Rotation" menuitem now works
  by selecting the next subtitle in order as listed
  in the menu.
Add informative comment

- Mostly a note to myself that I don't want to forget.
Make OSDLayers keep track of the text image

- To avoid doing the NSImage->CGImage conversion
  when it's not needed.
- I've had this one sitting around for a while now
  so it's possible this is a part of something
  bigger I was working on.
Get rid of the draw mutex in MMovieView

- Since CoreAnimation is handling the drawing
  we don't really need this. We have to do all the UI
  stuff on the main thread anyway.