Public headers should mainly include gdktypes.h, which already include the symbol visibility and versioning macros; we can also modify gdktypes.h to include the enumerations.
It was just added to avoid touching too many files. Touching them now.
This helper type will be used in the next commit for a new compute-size request on GdkDragSurface. It is analogous in spirit to GdkToplevelSize.