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/editwidgets.h

@@ -25,8 +25,8 @@
 
 #include "editcoordsys.h"
 #include "rtsurface.h"
-#include "../rtengine/coord.h"
-#include "../rtengine/rt_math.h"
+#include "rtengine/coord.h"
+#include "rtengine/rt_math.h"
 
 class ObjectMOBuffer;