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:
{F6952969}
in `/home/user/.config/fontconfig/conf.d/`

Settings UI:
{F7052980}

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