dataengines/weather: Simplify the alerts into a single list

Watches were only used by the envcan provider, which issues three types of alerts: 'warning', 'watch' and 'advisory' (https://www.canada.ca/en/environment-climate-change/services/types-weather-forecasts-use/public/criteria-alerts.html)

Instead of artificially splitting the alerts into two different lists of events (also missing the 'advisory' ones), remove the list of 'Watches', and combine them into just 'Warnings' with different priorities.

The dwd provider does this already, and most other providers (though still not implemented) also use a similar priority based system for their alert system.

A single list simplifies the models here and in the client side, and helps with an eventual port out of dataengines.

This, along the necessary changes in the applet ( kdeplasma-addons!522 (merged)) will allow to show alerts like this (previously FOG ADVISORY would not be shown):

The envcan web info for reference: