wayland_common : Support to hide cursor

Regard Qt::BlankCursor as hiding cursor. set_cusor(null surface)
means to hide cursor.

Change-Id: I868129472ab46651d1bb346928f6e3c337c12a32
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>
QtCompositor : Support to hide cursor

According to current spec of wayland protocol, set_cursor request with
null surface means to hide cursor.

Change-Id: I6f4d83d135fc55272468c475e31a28fb6c661fda
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>