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/dialog/sequence/sequence.h

@@ -28,6 +28,8 @@
 #include "project/item/sequence/sequence.h"
 #include "undo/undocommand.h"
 
+OLIVE_NAMESPACE_ENTER
+
 /**
  * @brief A dialog for editing Sequence parameters
  *
@@ -137,4 +139,6 @@ private:
   };
 };
 
+OLIVE_NAMESPACE_EXIT
+
 #endif // SEQUENCEDIALOG_H