Add current terminal window size to About popup

Description of suggested feature

It would be useful during reporting issues to know the size of the current terminal.

This is already available at runtime via urwid, since the popup code relies on it to determine which width (and maximum width) to use.

This entry should appear at the bottom of the 'Detected Environment' section of the 'About' popup.

Since the terminal size can often be changed while a popup is open, we will likely want to call this 'Current terminal size', though this can be adjusted later during a PR or after merging.