textfield height based only on clear text

Summary:
as the implicit height could change when switching to password mode,
use a textmetrics which always computes it from the visible text

BUG:399155

Test Plan: height doesn't change

Reviewers: Plasma, ngraham, rooty

Reviewed By: ngraham, rooty

Subscribers: sitter, rooty, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19599