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/editwindow.cc

@@ -18,7 +18,7 @@
 #include "editwindow.h"
 #include "editorpanel.h"
 #include "filepanel.h"
-#include "../rtengine/procparams.h"
+#include "rtengine/procparams.h"
 #include "options.h"
 #include "preferences.h"
 #include "cursormanager.h"