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/ApplicationSettings.h

@@ -82,8 +82,8 @@ public:
 	int getJavaMaximumMemory() const;
 	void setJavaMaximumMemory(int size);
 
-	std::string getMavenPath() const;
-	void setMavenPath(const std::string path);
+	FilePath getMavenPath() const;
+	void setMavenPath(const FilePath& path);
 
 	std::vector<FilePath> getHeaderSearchPaths() const;
 	std::vector<FilePath> getHeaderSearchPathsExpanded() const;