macosx: added an extra sanity check to fix video playback on Macs using the QT4 interface (fullscreen mode still broken)

 - Use inlines for expansion safety.

contribs: added an optional entry to compile QT4 on Darwin. run 'make .qt4_mac', if you want to build it. Note that you'll need to type 'yes' shortly afterwards to accept Qt's GPL. I'll provide a patch for this.

Qt4 - tracks detection is splitted from the other functions. Fix the "fullscreen button not showing" bug in some cases.

Qt4 - Fix compilation

Qt4 - Simple Preferences, audio: you expect to deal with % for the volume not abstract values from 0 to 1024... Added a label for that.

Qt4 - Update check every x days. Put it in the preferences, and in the privacy Dialog (may be useless on the privacy dialog ).

Qt4 - Launch the update every x days ( x being 14 days by default ). Check done just before the exec(). part 1.