Added apollo folder icon

[Deckstar]

Adding icons for an Apollo folder, because there is currently only an icon for and Apollo config file.

I love this extension very much, and I hope this little contribution can be useful! :)

Resolves #798

[PKief]

There's nearly no contrast between the default dark background of VS Code and the apollo folder icon. This is why is usually pick a lighter color for the icons:

I slightly changed the colors of all apollo icons to be more consistent:

[Deckstar]

Ah I see :) Sorry about that!

Anyway, the purple version looks fantastic. Thanks a lot!