Better app listing

[bertob]

This app's listing already looks pretty good, with just a few tweaks it could pass the Flathub metadata guidelines, which would make it eligible to be featured.

The issues are:

• The summary "⛅ Weather app for Linux made in LibAdwaita" is too long and a bit odd. Something simple that doesn't get into technical details, such as "Check the weather" would be better.
• The app needs brand colors https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#brand-colors
• The screenshot has non-default window decorations (should be close-only)

[SalaniLeo]

This will be resolved in #40 alongside other bugs

[bertob]

⛅ Check the weather!

This still doesn't meet the Flathub guidelines, in particular the one about non-standard formatting (the emoji and exclamation mark).

The MR also doesn't seem to change the screenshots or add brand colors, so unless that done somewhere else I think most of the stuff from the issue above still applies.