Get rid of relative include paths

* Use target_include_directories to specify include paths
* Specify project root (parent of rtgui and rtengine) as include path
* Replace relative includes with normal includes

rtgui/curveeditor.h

@@ -21,10 +21,10 @@
 #include "editcallbacks.h"
 #include "guiutils.h"
 
-#include "../rtengine/diagonalcurvetypes.h"
-#include "../rtengine/flatcurvetypes.h"
-#include "../rtengine/LUT.h"
-#include "../rtengine/noncopyable.h"
+#include "rtengine/diagonalcurvetypes.h"
+#include "rtengine/flatcurvetypes.h"
+#include "rtengine/LUT.h"
+#include "rtengine/noncopyable.h"
 
 class CurveEditorGroup;
 class CurveEditorSubGroup;