Release v3.5.1 (transitional)

This is a transitional release, which includes a new installer (based on MSI and not on an NSIS script) and some miscellaneous and minor changes. The version number has been kept at 3.5.1 for the time being.

Future updates will start from version 3.6 and will include a new configuration system and update checks.
Updates will be delivered here on GitHub.

Thanks to @andrewjmetzger for the MSI installer code.

Update: contains fix for issue #49, thanks to @Spasticon.