wayland: Fix drag grabs

We need to have the seat here. This makes weston-dnd not crash when
clicking on an item.
wayland: Pull in a fix for a FIXME from Weston
wayland: Use g_slice_new0

And fix a leak.
wayland: Fix destroying data sources

The resource is not embedded in the source, it's separate. We need
to get the user data here to not crash.