[Gtk3] fix gtk3 build and opacity feature flags for Windows

The recent Gtk3 version for Windows is 3.6 and lacks widget opacity/alpha support. Opacity feature should be  disabled, until a newer Gtk3 version is released for Windows.

Window opacity is supported on Windows and should be enabled.

This PR additionally removes gtk3 package information from the project file, to make it possible to build the Gtk3 backend on Windows.