Fix building without OTIO support

Added a new CMake flag that stops any OTIO related cpp files being
built.

Added extra #ifdefs to catch any OTIO related code in other cpp
files.

app/core.cpp

@@ -53,7 +53,9 @@
 #include "render/diskmanager.h"
 #include "render/pixelformat.h"
 #include "render/shaderinfo.h"
+#ifdef USE_OTIO
 #include "task/project/exportotio/exportotiotask.h"
+#endif
 #include "task/project/import/import.h"
 #include "task/project/import/importerrordialog.h"
 #include "task/project/load/load.h"