Keep GUI responsive during long tasks

[adamerose]

Currently the GUI completely freezes and becomes non-interactive during long tasks. Ideally PandasGUI should block the user from triggering more actions while one is running but still be movable and display a loading bar and a status message of what action is currently running.

Will need to put PandasGuiDataFrameStore actions inside QThreads and loop QEventLoop::processEvents() until it is finished.
https://doc.qt.io/qt-5/thread-basics.html#gui-thread-and-worker-thread

Need to make sure errors and stack trace logging are still handled gracefully