Fix certificate issues with upstream patches.

* https://hg.pidgin.im/pidgin/main/rev/9086eaeacd2c
* https://hg.pidgin.im/pidgin/main/rev/befb6523dc5c
Change the application name in Windows uninstall screen.

Parentheses are now used for the bitness part of the application name.
Detect the correct SignTool executable in Windows build.
Improvements for the build number in Windows build.

* Always recompile the about dialog.
* Remove the build date from the version-information resouce.

The build date is now informed only in the installer file and the about dialog, and should be the same in both.
Improvements for the Windows GTK+ bundle script.

* Remove the existing install directory before extracting the downloaded bundle.
* Remove the existing staging directory after creating a new bundle.

Removing the staging directory should avoid a problem where it somehow gets included in the installer.
Increase the height of the about dialog.