Dark theme support in our nightly builds

Thanks to the dedicated efforts of @mgrojo, our nightly builds now include a “Dark style” theme courtesy of the QDarkStyleSheet project:

To activate it, choose “Dark style” from the “Application Style” dropdown in Preferences→General:

To turn it off, choose “Follow the desktop style” instead.

The nightly builds for windows and macOS are here:

    https://nightlies.sqlitebrowser.org/latest/

The nightly builds for Ubuntu Linux are here:

    https://sqlitebrowser.org/dl/#nightly-builds-1