Text editor signals go to the focused widget

The effect is that selecting text in the error output
and using Edit->Copy copies the right text.
Set menu item sensitivies based on what is possible
Update docs to reflect focus changes
Configure font by name, not Pango calls
Make font editable

Changing the font affects all windows.  Presumably the
user doesn't want to change every window individually.
Marginally better 'best fit' implementation