Add a menu option to open the project directory

[vPumpking]

Add a menu item in Project menu that opens the project directory with the default file manager.

Use cases: It's rather time consuming and unintuitive to right click on res:// in ressource browser and choose open in file manager. I first added it on my custom version of godot, and found it so useful that I wanted to merge it.

As the change is very small, I made directly a PR.