Summary: As @ach pointed out, even after my last round of polish on the {nav Add Source} dialog, it was still too narrow, and many or even most URLs would not be fully visible in the text field. This patch remedies that, and now it can accommodate longer repo URLs without cutting anything off.

Test Plan:
with KDEApps Flatpak repo string, before:
{F5748786}

After:
{F5748787}

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: ach, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11219