refactor mss_qt move module imports to modules

[Reimar Bauer]

We introdruced mss_qt because of a short time we had pyqt4 and pyqt5 together.

The module import should become moved back where they are used, so that we as developer get our IDEs to use all help of autocompletion or documentation.

https://bitbucket.org/wxmetvis/mss/src/develop/mslib/msui/mss_qt.py#lines-138