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/graphics/QtGraphicsView.h

@@ -6,8 +6,6 @@
 #include <QGraphicsView>
 #include <QPushButton>
 
-#include "utility/file/FilePath.h"
-
 class QTimer;
 class QtGraphNode;