Implement choose project dialog and use for new-branch action

[jeremypw]

Fixes #1005

[jeremypw]

Maybe do not need both lines of text?

Maybe "git" should be main image with a "question" badge?

[danirabbit]

If we're creating a separate UI anyways, maybe we should stick this as a combobox on the headerbar to solve #991?

[jeremypw]

@danrabbit OK, but there are some design questions to answer:

• Where on the headerbar should it appear
• Should it hide or become insensitive if no git projects are loaded - or if only one project is loaded.
• Should it default to the first (or last) opened project if there are multiple projects but no documents opened?

[danirabbit]

@jeremy:

• My inclination would be to put it at the very start of the headerbar to imply that it effects every action in the application, including others that we may move to the headerbar
• Yeah, probably it would make sense to hide for now if there are no projects loaded, but it would make sense to have it when only one project is loaded since it still does indicate the active project.
• My inclination would be that we try to keep it updated with the most recently active project. Basically whatever our guess would have been before, we could do that logic here pro-actively instead of waiting until an action is taken. So I guess we would want to update that when we change the focused document?

[jeremypw]

Closing in favour of #1023