webdav: remove unused defines

This are leftovers from before the gnome-user-share binary was split in
two.

https://bugzilla.gnome.org/show_bug.cgi?id=739872
webdav: remove unused code

https://bugzilla.gnome.org/show_bug.cgi?id=739872
webdav: use GApplication for unique instance

Instead of getting an X selection. Currently, the obexpush and webdav
helpers compete for the same selection, so it's impossible to run them
both at the same time.

https://bugzilla.gnome.org/show_bug.cgi?id=739872
obexpush: make obex_agent_up() and obex_agent_down() symmetric

Make these two functions idempotent and symmetric in return value;
obex_agent_up() cannot return NULL since obex_agent_new() won't ever
return a NULL GObject.

https://bugzilla.gnome.org/show_bug.cgi?id=739872
obexpush: use GApplication for unique instance

Similar to what we just did for webdav, stop using an X selection to
ensure an unique instance of this.
Also, cleanup the code a little bit in the process.

https://bugzilla.gnome.org/show_bug.cgi?id=739872