Add Eq instances for all of Graphics.UI.Gtk.Gdk.Enums

[guest]

None of the enumerations in Gdk.Enums are instances of Eq, which makes it cumbersome to check for certain events. For example, I wanted to keep track of the fullscreen state of a window with the following code:

ref <- newIORef False
onWindowState win $ \WindowState { eventWindowState = state } -> do
    writeIORef ref (WindowStateFullscreen `elem` state)
    return True

However, writing the code this way requires an Eq instance; workarounds require either manually defining one or using pattern matching, which is cumbersome because state is a list of WindowState values.

It would be trivial to add deriving (Eq) to these enumerations (and also Read/Show, for debugging purposes).

[axel]

Done. I didn't add Read since I think this incurs quite a lot of code with no real benefit.