"Help us improve Apertium!" link doesn't say how

[12people]

The current "Help us improve Apertium!" link points to the About dialog, which doesn't point the user to any way to contribute. It'd be useful to link to e.g. https://wiki.apertium.org/wiki/Contributing in another window, create a special dialog centered around contributing, or modify the About dialog to make it clear how one can contribute.

[flammie]

this is same as #405