Display the GUI version somewhere

[arvoelke]

It would be nice if I could go nengo --version and/or see the version displayed somewhere in the GUI (e.g., next to the "Help" or "?"). This would help me know if it's properly picking up the dev version, a version from pip, etc.

I can go:

python -c "import nengo_gui; print(nengo_gui.__version__)"

but this won't tell me what the nengo executable points to -- only what python happens to find -- which seems to depend on which was installed latest?

At the same time, the core nengo version could be displayed in the same location, and even the version of each backend that it can see!