Always show Tree View

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

The Tree View isn't created if no projects are open in a window, limiting discoverability of how to add a project. To make it easier to add projects, this PR now always shows the Tree View. If no projects are open, the Tree View prompts to add projects:

Alternate Designs

None.

Benefits

Easier to add projects directly through the Tree View.

Possible Drawbacks

Tree View will now appear, even when no projects are open.

Applicable Issues

Refs #1313. There currently is no way to "promote" a single file to a full project without manually selecting its project root.
Closes #1312
Fixes #68

Todo

• A lot of the styles for the new view were copied over from the GitHub package. Is there a way to improve on that situation, somehow...?
• Some potential functionality-changing behavior with regards to consuming file icons. I did this so that updateRoots wasn't called twice in a row on startup, which sometimes led to two of the add-project-views being added to the DOM.

[nathansobo]

@50Wliu The code and test coverage look good, but I adjusted the wording a bit and got rid of the H1. I want to make it clear that a workspace is always associated with a project, but sometimes that project is empty. You can add folders to it, or you can reopen a previous project. I played with the second button being "Reopen A Previous Project" but it felt a little long. What do you think? Happy to iterate on this.

[winstliu]

Looks great to me! Thanks :)

[nathansobo]

Thanks for doing this. It's a really nice UX improvement.

[gianmichelemariani]

Not really a huge fan of this, especially when opening single files.
Can at least add an option in the preferences so that it can be disabled?

[pneedle-rh]

Not really a huge fan of this, especially when opening single files.
Can at least add an option in the preferences so that it can be disabled?

I completely agree with this request to include an option to disable this feature. I have created this RFE.