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_gui/qt/view/QtBookmarkView.cpp

@@ -250,7 +250,8 @@ void QtBookmarkView::showBookmarksClicked()
 void QtBookmarkView::setStyleSheet()
 {
 	m_widget->setStyleSheet(utility::getStyleSheet(
-		ResourcePaths::getGuiPath().concat(FilePath("bookmark_view/bookmark_view.css"))).c_str());
+		ResourcePaths::getGuiPath().concatenate(FilePath("bookmark_view/bookmark_view.css"))
+	).c_str());
 }
 
 void QtBookmarkView::refreshStyle()