Allow to edit environment path in selection dialog manually

[krassowski]

I was annoyed by this limitation for quite some time (especially because of lack of support for multiple instances #476). #477 also brought this point:

Before I opened the file browser, I wondered why I couldn't paste the path from my clipboard directly into the field with an orange border. It looks like a text input field by the way of having an I-beam cursor when moving the mouse over it, but activating it and hitting command-V did nothing. If it's not a text input field then maybe it should be styled differently.

For it is the difference between substituting a_name in ~/.pyenv/versions/a_name/bin/python for b_name (super quick) vs navigating two directories up, and then two directories down (6 clicks more).

No changes are needed on scripting logic.

[mbektas]

LGTM!