logic: reduce access to filesystem while indexing

* use canonical filepath cache in cxx indexer
* made constructors of FilePath explicit
* use FilePath at more places instead of string
* forward declares FilePath wherever possible

src/lib_gui/qt/view/graphElements/QtGraphNodeData.h

@@ -1,10 +1,10 @@
 #ifndef QT_GRAPH_NODE_DATA_H
 #define QT_GRAPH_NODE_DATA_H
 
-#include "utility/file/FilePath.h"
-
 #include "qt/view/graphElements/QtGraphNode.h"
 
+class FilePath;
+
 class QtGraphNodeData
 	: public QtGraphNode
 {