Add support for _NET_WM_STATE_DEMANDS_ATTENTION.

Add support for _NET_WM_STATE_DEMANDS_ATTENTION.

_NET_WM_STATE_DEMANDS_ATTENTION indicates that some action in or with
the window happened. It's a weaker hint than urgency flag of WM_HINTS,
but some applications and almost all Qt applications use it instead of
WM_HINTS' urgency flag (one example is Skype).