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/window/QtSelectPathsDialog.h

@@ -2,8 +2,8 @@
 #define QT_SELECT_PATHS_DIALOG_H
 
 #include "qt/window/QtTextEditDialog.h"
-#include "utility/file/FilePath.h"
 
+class FilePath;
 class QListWidget;
 
 class QtSelectPathsDialog