gdk::WindowAttr should contain an EventMask in the event_mask field

[tsahyt]

Currently the event_mask field has type i32. The EventMask struct exists and can be built as a bitfield, and then converted using .bits(), yielding an u32 which then needs to be cast in order to be used in the event_mask field. This is somewhat unwieldy, and it would likely be preferable to just have the WindowAttr struct contain the EventMask.