feat(web): specify icon

[sevenautumns]

Some favicons are only provided with a resolution of 16x16, resulting in the logo being rather muddy inside of pop-launcher.

WIth this pull-request some changes were made.

• The Definition-Struct was extended with the icon-variable, which does not need to be defined.
  • If it is defined, it is prioritized as a candidate for the downloaded icon.
• The cascading fallback icon download match-cases were replaced with a List of icon sources.
  • The first successful icon of this list is used.

As an example Nix Packages can be used

(
   matches: ["np"],
   queries: [(name: "Nix Packages", query: "search.nixos.org/packages?channel=unstable&query=")]
),

With this a rather muddy looking icon is used. But should we use this instead:

(
   matches: ["np"],
   queries: [(
      name: "Nix Packages", 
      query: "search.nixos.org/packages?channel=unstable&query=",
      icon: "https://nixos.wiki/nixos-logo-small.png",
   )]
),

A more crisp looking icon is used instead.

I am not to proud of the for-loop, processing the icon_sources-list. The alternative would be to convert the icon_sources-list to a futures stream, but this introduced some problems and made it less obvious what is happening.