feature: 'save as' button for existing projects

[skorasaurus]

If I want to modify an existing project and save it under a new name, the current workaround would be to copy the existing project's folder in a file manager/terminal, rename it, then reopen in tm2.

A 'save as' feature to rename the projects folder while preserving the original project's name and files would be nice.

I ran into this while customizing the osm-bright tm2 project for my own needs.

[bengolder]

Just tried using the existing 'Save As' dialog when opening an existing project, and it could really use more of the standard file save dialog options. For example, there doesn't appear to be a way to create a new folder.