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/tonecurve.cc

@@ -26,8 +26,8 @@
 #include "eventmapper.h"
 #include "options.h"
 
-#include "../rtengine/procparams.h"
-#include "../rtengine/utils.h"
+#include "rtengine/procparams.h"
+#include "rtengine/utils.h"
 #include "editcallbacks.h"
 
 using namespace rtengine;