New Branch Dialog: offer a dropdown if focused project is unknown

[danirabbit]

Problem

When creating a new branch, if Code isn't able to determined the focused branch we get a warning dialog explaining that we must first open a project etc.

Proposal

Instead, we could offer a dropdown listing the available projects so that the user could choose the one they meant to create a branch for

[jeremypw]

We probably want a separate dialog to do this as the same issue applies to virtually every project-specific action with a keyboard shortcut (e.g. Global Search, New Branch)