Store the cursor name on the cursor (rather than always using its type).
Use this when setting a cursor on a surface.
The mir server will fallback to using standard cursors from the cursor
theme if the name used is not one of those defined by mir, which is more
or less what we want to happen here in case of creating a cursor by
name.
The Mir backend was checking for button mask changes to generate the appropriate
GDK event. When Mir generates a touch event it has no button mask. In this case
we'll just generate a primary button event.
