Show tooltip with full URL for URL button with overridden text

Summary:
Normally the URLButton shows the full URL in its text. However if this text is overridden
to anything else, then the user can't know where the URL points, which may be worrisome,
and could be abused by malicious applications to obscure a hostile URL.

This patch implements a tooltip that shows the full URL when the text property has been
overridden and no longer shows the real URL.

Test Plan:

Reviewers: VDG, Kirigami, mart, apol, ndavis

Reviewed By: VDG, Kirigami, mart, ndavis

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D27663