Remove dependence on modules: QtWidgets, QtGui

[zhang-peter]

The QCoreApplication class provides an event loop for basic Qt applications. QApplication is not necessary for every application.

QGuiApplication and QApplication can be used as the following way for some Gui Applications.

app = QApplication(sys.argv)
loop = QEventLoop(app)
asyncio.set_event_loop(loop)

[pohlt]

After applying the changes in this PR and PR #88 quamash now works with PySide2, so I would strongly support the integration of both PRs.

[harvimt]

Included in #111