src: implemented move constructor/assignment for filepath

* improves indexing performance by roughly 5%
* switched to use uniqu_ptr for boost path to get rid of including boost in header
* added different methods to change the current FilePath or to make a copy and change that one
* used these methods appropriately throughout the code base
* removed exists() method from FileSystem because it is already contained in FilePath
* added const in some places

src/lib_cxx/data/parser/cxx/utilityClang.cpp

@@ -120,7 +120,7 @@ std::string utility::getFileNameOfFileEntry(const clang::FileEntry* entry)
 		}
 		else
 		{
-			fileName = FilePath(entry->getName().str()).parentDirectory().concat(FilePath(FilePath(fileName).fileName())).str();
+			fileName = FilePath(entry->getName().str()).getParentDirectory().concatenate(FilePath(FilePath(fileName).fileName())).str();
 		}
 	}
 	return fileName;