Fix PrivateActionToolButton behaviour with showText vs IconOnly

Summary:
PrivateActionToolButton behaves slightly different when setting
showText to false versus setting display to IconOnly. This is
because the implicitWidth binding does not check the display
property. This fixes that to make sure both showText: false
and display: IconOnly return the implicitHeight, making the
button square in both cases.

Test Plan: Kirigami Gallery still works correctly.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D27698