Add support for settings portal

Summary:
Sandboxed applications usually don't have access to read kdeglobals configuration. For this
reason we introduced Settings portal, to be able to obtain most necessary configuration without
allowing applications access to hosts configuration. The Settings portal implementation in
xdg-desktop-portals-kde was develop mostly for plasma-integration needs so it allows to get
colors, fonts, widget theme and some other configuration from kdeglobals.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17213