libwnck: update to 2.31 / avoid conflicts with libwnck3

Both libwnck and libwnck3 install the commandline tools 'wnck-urgency-monitor'
and 'wnckprop' to bindir. The packages depending on libwnck were checked: In
none of them there is a trace for these tools.

To avoid conflicts the tools are installed by libwnck3 only. If something
requires 'wnck-urgency-monitor' or 'wnckprop' it needs to install libwnck3.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>