sending an icon path. This doesn't work for symbolic icons.

requested by an app.  The icon and visibility state aren't necessarily set
at the same time.

Reproduced with kdeconnect.

The state can decide which icon to use, so if it changes, the icon
may also, even if we haven't gotten an explicit icon changed signal.

lookup key.

When a new item tries to register, its key is added to the item table
with a null value, to prevent trying to register the same client
multiple times. When registration is finished, the key is removed,
and re-used to add the new item to the table.

If the app disappears before that registration finishes, we remove
its key from the table (handle_sn_item_name_owner_lost), so when the
item registration completes (it will because a proxy doesn't need an
owner), our steal/re-add fails because the key was removed already.

This will have us steal the key early when completing registration,
and abort if it's null.

Also wrap g_object_unref when removing an item from the table to
prevent a warning.