Change repo dialog looks weird

[marusak]

repo name is too small, add button is weirdly aligned, the row is gray...

[garrett]

Duplicate of #176, where I proposed mockups.

#176 (comment) and a few below that comment specifically have some revised mockups that match what we do in the PackageKit version of the software updates page. (Ignore mockups above the linked comment, as those are old experimentations, before we worked on redesigning & implementing PackageKit long ago.)