Adds writeAgain to Socket, which tells it to emit readyWrite whenever
possible. (for IDoWrite and TLS)
Builds GnuTLS, TagLib and ImageMagick on Linux (not included).
Removes winsock2 from TLS on Windows.
Displays the plugin loading error as a log Error instead of a Debug.
Uses qobject_cast instead of dynamic_cast in TagLib::Preview.