Use shorter names for constants, expand exports of constants

[timholy]

Plus a couple of random fixes

[vtjnash]

Do we want to drop the _MASK also?
e.g. GdkEventMask.POINTER_MOTION_HINT instead of GdkEventMask.POINTER_MOTION_HINT_MASK

[timholy]

I wondered about that, too. I'll add that on.

[timholy]

Now I remember why I didn't do this initially. GdkEventMask makes it clear that the constants are used as a mask. In contrast, GdkModifierType doesn't make it clear that the constants are bitmasks, except via the _MASK as part of each constant's name. We could rename the baremodule GdkModifierTypeMask, I suppose. But then it disagrees with the naming in libgdk.

My view is we can still drop the _MASK and not worry about it, but I thought I should mention this concern.

[vtjnash]

I guess it didn't seem that important to me to identify that they are a mask. Eventually, (once JuliaLang/julia#2988 merges), I want to type these as Enum and Flag (e.g. Mask) Julia types.

[timholy]

Agreed. Done.