Rewrite project template picker in Hammer

[uliwitness]

We used to (until 2020-05-30) have a window that let you select from several template projects to create when you chose "New Project".

This was deleted because Apple broke NSCollectionView's old API. No use rewriting it for the new API if we could instead put the time into adding collection view support to Stacksmith itself and write the template picker as a Stacksmith Stack in the Home stack.

Desired features:

• Reflects the folder hierarchy in the "Project Templates" folder of Stacksmith.app (at least 1 level deep, don't think we need more)
• Lists all stacks in there, with each as a nice icon
• possibly could support custom icons that reflect the stack, or first-card previews
• possibly could show a short description of what the template is for in a separate pane under the list
• Creates a new project by copying the file the user selects and clicks "OK" on, or allows them to cancel if they decide otherwise.

[uliwitness]

This would be a good first issue for someone once Stacksmith has all the features needed. We have directory listings, but we can't yet copy files.