applets/weather: Warn about missing capabilities of the selected provider

Make it more clear to the users that some providers (mainly wetter.com, but also dwd for some locations) have missing capabilities and will not show the current conditions.

Ideally we would not offer providers with these limited capabilities, but we still don't have an alternative for many of the locations on our worldwide user base.

This showed up in bugs like https://bugs.kde.org/show_bug.cgi?id=518137 or https://bugs.kde.org/show_bug.cgi?id=524478 (and duplicates)

Station Config	Panel Appearance Config