display: Remove dummy manage implementation
xdmcp-chooser-slave: Don't fetch the display_id

We don't need it.
display: Remove gdm_display_get_bus_connection

It's unused.
xdmcp-display: Remove dummy skeleton

This doesn't do anything, so why are we keeping it around?
flexiserver: Remove a dead branch
daemon: rename gdm-slave-proxy to gdm-slave-job

It doesn't actually proxy calls to the slave, so it's misnamed.
utils: move login screen jumping code to common

flexiserver has a bunch of complicated logic i want to use in the
worker.  Move that code to the common library and make flexiserver
link against the common library.
flexiserver: don't open a display

The only reason it uses gtk is to show some awful dialogs in the case
of failure.  Drop that code.

Now gdmflexiserver can be run from the command line.
daemon: rename slave_bus_proxy to slave_proxy

The _bus_ middle part of the name was only there to prevent ambiguity
with the, then named "gdm-slave-proxy", but now named, gdm-slave-job class.