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/lockablecolorpicker.cc

@@ -19,10 +19,10 @@
 
 #include "lockablecolorpicker.h"
 #include "options.h"
-#include "../rtengine/color.h"
-#include "../rtengine/improcfun.h"
-#include "../rtengine/rt_math.h"
-#include "../rtengine/utils.h"
+#include "rtengine/color.h"
+#include "rtengine/improcfun.h"
+#include "rtengine/rt_math.h"
+#include "rtengine/utils.h"
 #include "imagearea.h"
 #include "multilangmgr.h"
 #include "navigator.h"