there's no reason for window size computation to be linked to the

there's no reason for window size computation to be linked to the
ProgressMeter, so make it available from state.
- computation is lazy, so no runtime difference;
- don't bother setting a SIG{WINCH} on non-tty;
- progressmeter already has access to state;
- do an empty window_size_changed in base state, override it for
addcreatedelete to inform the progressmeter.

to be reused in dpb...