Deleted projects still show on GUI's project list

[chiunhau]

.p5rc won't update itself when someone delete projects. So it still tracks those already not exist. There are some possible ways to solve this:

• Show hints on error, asking users to update their .p5rc manually.
• Provide a command like p5 delete PROJECT_NAME to remove specific project folder and update .p5rc as a callback.
• Get rid of the reliance on .p5rc, and create a new way to track active projects.