[LightDM] Qt LightDM

[David Edmundson]

I've been rewriting a large amount of the Qt library, it now acts as a
wrapper round the GObject lib rather than duplicating all the code.
This means it now works again, as well as staying up to date wth all
the latest fixes on the main branch.

The code has now been rewritten to be future-proof for any upcoming
changes and obeys normal Qt library conventions.

Warning:
This breaks API and ABI compatibility with the previous QLightDM. As
far as I know no-one is using this version of the library except for
me.

The gobject version of the library is completely unaffected.

If anyone wishes to review or has any objections, please speak now.

David