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/QtTooltipView.cpp

@@ -31,7 +31,7 @@ void QtTooltipView::refreshView()
 	m_onQtThread([=]()
 	{
 		m_widget->setStyleSheet(
-			utility::getStyleSheet(ResourcePaths::getGuiPath().concat(FilePath("tooltip_view/tooltip_view.css"))).c_str()
+			utility::getStyleSheet(ResourcePaths::getGuiPath().concatenate(FilePath("tooltip_view/tooltip_view.css"))).c_str()
 		);
 	});
 }