improve dialog for removing repository

[sgratzl]

so far the "remove" button is a regular button right before the "back" button. It happened to me more than once that I click on it by accident:

maybe a confirmation dialog or and advanced menu would be an alternative solution

[jsmnbom]

A simple yes/no confirmation does indeed seem like a good idea.

PRs are very welcome, otherwise I will probably get around to it sometime :)