Update Deprecated LightDM Calls

[codehearts]

Apparently lightdm-webkit2-greeter has really changed its API. I'm going to need a new mock (https://github.com/CytoDev/LightDMMock) and possibly some more framework code

[codehearts]

People are starring this theme, so I assume they're using a popular distro with an older version of the greeter. Ideally this theme should work with the old and new API, see if that's feasible