Event accessors

[gkoz]

We have the following goals for the high level APIs like these getters:

• Do not expose raw pointers. We have helper function for common cases, you might want to look at the surrounding modules and glib::translate for guidance (example of returning a window). It seems likely that most pointers carried by the events are nullable, so the type would be wrapped in Option (the helper functions can handle that).
• Do not expose any of the FFI types (or at least type names) that have higher-level counterparts. If an FFI type hasn't been reexported yet, add it to lib.rs among the others. In particular,
  • GdkRectangle is currently exposed as cairo::RectangleInt;
  • strings might translate to &str or Option<&str> or Cow<str> depending on our assumptions, new helper functions will be needed for that.
• Integers that aren't expected to be negative (e.g. width, height) should be converted to u32 with assertions ([RFC] Integer signedness story gtk#246, we don't have a settled style for that yet).
• There's a still open question of whether to make keyval type an enum: Export GDK_KEY constants as u32. #105
• Ideally, you should have some idea of how the event can be used. For instance, I'd rather not bind EventProperty at all yet because the properties story is not clear.

[gkoz]

This issue has served its purpose it seems.