Update KDE Plasma Desktop to 5.18.4.1

Tuesday, 31 March 2020. Today KDE releases a bugfix update to KDE Plasma 5,
versioned 5.18.4. Plasma 5.18 was released in February 2020 with many feature
refinements and new modules to complete the desktop experience.

This release adds a month's worth of new translations and fixes from KDE's
contributors. The bugfixes are typically small but important and include:

• [Image Wallpaper] Fix thumbnail generation when model is reloaded in-flight.
• [applets/systemtray] Clear item from shown/hidden list when disabling entry.
• [GTK Config] Construct font style by hand instead of relying on Qt function.

Changelog:
https://kde.org/announcements/plasma-5.18.3-5.18.4-changelog