Improve Add button behaviour.

The add button in the Headerbar is short of terrible atm from a UX perspective. I've gathered some of my notes and thoughts on how it could be improved.

Any input on the subject would be appreciated.

Feel free to work on it and open Merge Requests implementing the suggestions or other improvements that you thing would be useful.

Things that could be done:

• Move from using a label for showing errors to the GtkEntry warning cone widget
• Parse Url::Parse errors and return relevant suggestions on how to solve them
• Make it so the url hasn't have to start with http:// or https:// etc
• Restrict it to allow http/s urls to be added, instead of let's say ftp://foo.bar etc
• Add an example url when the entry field is empty
• After a feed is added, instead of clearing the url just select it.
• Refactor the currently messy code.