[applets/weather] Use unit description from KUnitConversion

Summary:
Use unit description directly from KUnitConversion instead of making a new string.
This avoids duplicating translation strings and improves consistency.
The units name is now description (symbol) also for consistency with krunner

Test Plan:
No behavior changes within the applet
Configure -> Units shows translated description (symbol) for every unit

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24953