[applets/weather] Port from KIconLoader to QIcon

Summary:
KIconLoader was used only to check if the iconName is valid.
This patch uses QIcon::hasThemeIcon() instead.
It might help T11637 a little.

Test Plan: No behavior changes

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D24964