Edit Profile Dialog: Fix "Smooth fonts" preview

Summary:
setStyleHint() resets styleStrategy (which has information about
font smoothing) when the second argument is missing.

Test Plan:

• Settings → Edit current profile... → Appearance
• Unselect/select "Smooth fonts" checkbox

Actual result: text rendering in the main terminal window does not
change.
Expected result: text rendering in the main terminal window should
use smoothing or not, depending on the checkbox state. The option change
should be visible instantly, without clicking OK or Apply.

Reviewers: Konsole, hindenburg

Reviewed By: Konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: Konsole

Differential Revision: https://phabricator.kde.org/D16361