[arch-general] LightDM GTK+ Greeter 2.0.0

Hi all, Version 2.0.0 of the LightDM GTK greeter was released about week ago. This release drops GTK2 support. Therefore I propose to rename the current "lightdm-gtk3-greeter" into "lightdm-gtk-greeter" and drop the legacy "lightdm-gtk2-greeter" to AUR. I could keep "lightdm-gtk2-greeter" around for a while in [community] if some people want me to (I'm thinking people using MATE for example, but it's going towards GTK3 anyway), but keep in mind that you can use LXDM instead if you really need a GTK2 display manager. If there aren't any objections by the end of the week, I'll proceed with my proposal. Cheers, -- Maxime