Add preferences-desktop-theme-applications and preferences-desktop-theme-windowdecorations icons

Summary:
Adds an application style and window decorations theme icon and adds a symlink.

Test Plan: Search for "preferences-desktop-theme" in Cuttlefish.

Reviewers: VDG, ndavis

Reviewed By: VDG, ndavis

Subscribers: ngraham, ndavis, kde-frameworks-devel, VDG

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19733