ui: Hide nodes and edges from graph via context menu action or Alt + Click (issue #472)

* show all context menu actions, but disable unavailable
* added hide action to undo redo stack

src/lib_gui/qt/utility/QtContextMenu.cpp

@@ -54,6 +54,9 @@ void QtContextMenu::addFileActions(FilePath filePath)
 {
 	s_filePath = filePath;
 
+	s_copyFullPathAction->setEnabled(!s_filePath.empty());
+	s_openContainingFolderAction->setEnabled(!s_filePath.empty());
+
 	addAction(s_copyFullPathAction);
 	addAction(s_openContainingFolderAction);
 }