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:
* {nav 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