Add WinSparkle to source code bundle in Windows build.
Drop Bonjour SDK in Windows build.

The following upstream patches have been merged in order to eliminate the dependency:

* https://hg.pidgin.im/pidgin/main/rev/7012c84d4d3b
* https://hg.pidgin.im/pidgin/main/rev/cd5a6082cb27
Update the detailed changelog.