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/settings/Settings.cpp

@@ -31,7 +31,7 @@ bool Settings::load(const FilePath& filePath)
 {
 	if (filePath.exists())
 	{
-		m_config = ConfigManager::createAndLoad(TextAccess::createFromFile(filePath.str()));
+		m_config = ConfigManager::createAndLoad(TextAccess::createFromFile(filePath));
 		m_filePath = filePath;
 		return true;
 	}