GTK Accessibility relations

[ZettaScript]

When using a screen reader (e.g. Orca), you can only navigate between the form items, but cannot read the labels. This is a problem for visually impaired people.

Accessibility relations can be added so that the screen reader known which label corresponds to what focusable item.

I'll try to make a PR about this. I know how to do it with GTK but don't have experience in Vala yet.

[ZettaScript]

In fact the version 0.4.1 is partially accessible but master is completely not: Orca says nothing. I'll see how to fix it...

Edit: It turns out granite has accessibility issues. https://github.com/elementary/granite/issues/536