Implement syncing of theme preferences between SDDM and Plasma

Summary:
This patch exposes UI options in sddm-kcm which enable users to sync their theme preferences used in Plasma with SDDM.

Syncing of UI font, font rendering, color scheme, desktop and icon theme has been implemented.

A reset button has also been added in order to allow easy removal of all SDDM syncing-related customization.

Test Plan:
For testing a different font, you need to have a file like this one:

Settings UI:

Reviewers: Plasma, ngraham, davidedmundson, VDG

Reviewed By: ngraham, VDG

Subscribers: leinir, cfeck, GB_2, ndavis, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D22191