[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