applets/weather: Refactor and Visual improvements of the Notices view

Refactor the Notices model and view

Simplify the notices model into a single list model, where the notice type is added as a property.

Now we can use a proper ListView with actual headers and a scrollbar. This fixes a historical issue where unwrapped text could make the applet wider.

Visual improvements for Warnings

1. Improve the layout of the notices (warnings and watches), as a formatted tabbed list showing also their timestamp
2. To access the expanded information link, use an information button instead of showing all the text as a link, which might make the text more difficult to read. Also hide the button when no link is provided.
3. Add a Warning icon for the Notices tab, to make it more attention grabbing as these are usually emergency warnings.

BEFORE	AFTER	AFTER (env. Canada)

4. use Kirigami.ListSectionHeader for the headers Remove the sections header (Depends on plasma-workspace!3735 (merged))
5. Show flags for event priority

Fortunately (for locals, not for debugging) is not that easy to trigger the Notices tab, so a tip for testing: go to https://www.dwd.de/DE/wetter/warnungen_gemeinden/warnWetter_node.html or https://weather.gc.ca/?layers=alert#wb-cont, and pick a place which is under some weather warning.

Depends on !519 (merged) and !520 (merged)