Global headers

[timsuchanek]

As mentioned here #104, the "reuse headers" option is already nice, but even better would be "global headers".

We need a design for this, but it could add a lot of value when you for example need to set auth headers for all tabs.

[timsuchanek]

These headers could live in the global settings, however, they should be individually configurable per project. So having an extra config for this in the playground probably makes more sense.