Hide Add Python Project context menu item if already selected as Python Project

[cwebster-99]

1. Select a folder/file as a Python project
2. Navigate to the file explorer
3. Right click on workspace or file that is already added
4. ❗We still show "Add Python Project" even though it is already selected

Can we hide this if it is already selected to reduce confusion. I think renaming this item to "Add as Python project" would be helpful as well since we plan to introduce project creation this could be confused as creating a new project from scratch.

[eleanorjboyd]

To test, make sure that:
When user right-clicks on a file/folder not yet added as a Python project: "Add as Python Project" is shown
When user right-clicks on a file/folder already added as a Python project: The menu item is hidden