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/controllines.h

@@ -21,7 +21,7 @@
 #include <memory>
 
 #include "editcallbacks.h"
-#include "../rtengine/perspectivecorrection.h"
+#include "rtengine/perspectivecorrection.h"
 
 class Circle;
 class Line;