various: encapsulate all code in the olive namespace to avoid name collisions

Large-scale code cleanup. Also adds a license to the top of all files that were
missing it.

app/widget/playbackcontrols/playbackcontrols.h

@@ -30,6 +30,8 @@
 #include "dragbutton.h"
 #include "widget/slider/timeslider.h"
 
+OLIVE_NAMESPACE_ENTER
+
 /**
  * @brief A playback controls widget providing buttons for navigating media
  *
@@ -132,4 +134,6 @@ private slots:
 
 };
 
+OLIVE_NAMESPACE_EXIT
+
 #endif // PLAYBACKCONTROLS_H