Fix the Add Source dialog

Summary:
Fix a variety of bugs and usability issues with the 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:

After:

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 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