Increase the "Add Source" dialog's width

Summary: As @ach pointed out, even after my last round of polish on the 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:

After:

Reviewers: apol, Discover Software Store

Reviewed By: apol, Discover Software Store

Subscribers: ach, plasma-devel

Tags: Plasma

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