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/data/TaskCleanStorage.cpp

@@ -2,6 +2,7 @@
 
 #include "component/view/DialogView.h"
 #include "data/PersistentStorage.h"
+#include "utility/file/FilePath.h"
 #include "utility/scheduling/Blackboard.h"
 #include "utility/utility.h"
 #include "Application.h"