I forgot to multiply the requested size by the scale factor when
checking to see if the icon needs to be scaled after getting it from
libwnck.

Which automatically set GLIB_VERSION_MAX_ALLOWED.

Disconnecting in finalize is too late; we can still get signals from the
window after the menu item widgets are destroyed.

...to fix intermittent failure to update title.

Fixes: issue #3

We need to tell libwnck the default icon size we want, otherwise it'll
give us icons of its default size, which are quite small.  Since we
don't want to keep invalidating the entirety of libwnck's icon cache any
time an app wants an icon of a different size, we just track the largest
icon size ever requested, and tell libwnck to generate icons of that
size.  If the caller requests something smaller, we downscale.

This isn't perfect, but is probably the best we can do.

Also add a couple missing symbols

Prior to this changed, the state-changed signal passed the old state to
the signal handler.  The other signals that handle bitfield changes
instead pass a changed mask and the new state.