Refactor 3D settings, improve playback handling and integrate into skins

This PR adds a central class called StereoscopicsManager which holds generic stereoscopic related functions to be used by video players and other classes. It also contains the most of the logic for stereoscopic related user interactions (changing settings, dialogs, ...) and performs the necessary tasks (switching GUI into 3D mode).

It also adds a central toggle to enable/disable stereoscopic features as well as several playback related options:
- define a preferred playback mode for stereoscopic movies (ask, preferred stereoscopic mode, ignore/don't to anything automatically)
- specify a preferred stereoscopic mode in which according movies should be rendered/converted to
- turn off stereoscopic mode if video playback stopped

The last commit is my attempt to integrate those features nicely into the video OSD, so that they can be controlled/changed via remotes.

edit: Note that this won't compile on OSX/iOS yet because I didn't know how to add the StereoscopicsManager.cpp to the xcode projects. Feel free to create a PR on my branch with according changes and I'll add them to this PR.