Better appdata summary

[bertob]

The current summary "Extract the dominant colors from any image." could be better, it's a bit too long and has a period at the end.

I'd recommend something like "Extract color palettes from images" or "Generate color palettes".

More general guidance here: https://gitlab.gnome.org/GNOME/Initiatives/-/wikis/App-Metadata

[nate-xyz]

26f1577