build: Enable dleyna plugin in distcheck

https://bugzilla.gnome.org/show_bug.cgi?id=733253
help: Add mention of DLeyna plugin

https://bugzilla.gnome.org/show_bug.cgi?id=733253
plugins: Remove UPnP plugin

It's been replaced by the DLeyna-based plugin

https://bugzilla.gnome.org/show_bug.cgi?id=733253
dleyna: Remove constant warnings when browsing

Properties requested by grilo but not support by DLeyna produced a
warning. For every item!

https://bugzilla.gnome.org/show_bug.cgi?id=733253
dleyna: ChildCount is an int32, not a uint32

As verified inspecting the dleyna-server source code.

https://bugzilla.gnome.org/show_bug.cgi?id=733253