Enhance snippets with configurable color/namespace for packages

[Janghou]

Myriad of snippets added by multiple packages will create confusion when typing the prefix.

Different colours can clear that up, so you can easily spot which to which package the prefix / snippet belongs. Like doing with providers and completion, but then with the snippet icon.

CON: colliding with themes
Maybe just a border and not complete background.

[Janghou]

Let color visualize namespacing.

At the moment snippets from different packages with the same prefix and scope collide, and I think only the first is shown. The second is not available!
Namespacing/coloring can help. Also reference #43.