Fix the Add Source dialog

Summary:
Fix a variety of bugs and usability issues with the {nav Add Source} dialog:
- Make the close button actually close the dialog
- Give the text field focus so you can immediately start typing
- Make the return key push the Add button
- Remove needless close symbol in top-right corner
- Give the buttons icons
- Re-word the title and make it larger

BUG: 390148

Test Plan:
Before:
{F5738465}

After:
{F5738429}

Tested with Flatpak backend:
- Invalid URLs are rejected
- Valid URLs that nonetheless don't point to a Flatpak repo trigger an error message
- The close button now closes the dialog
- The escape key still closes the dialog
- The return and enter keys press the {nav Add} button

Reviewers: #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: acrouthamel, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11003