Manually resize KCMUtilDialog to sizeHint()

Summary:
Workaround for https://bugreports.qt.io/browse/QTBUG-3459

Currently adjustSize() is limited to 2/3 of the screen size for windows
This adds unnecessary scrollbars on dialogs that would otherwise fit the
screen
Manually resize the window after adjustSize() happens to avoid this
limitation

CCBUG: 389585

Test Plan:
Tested on a 1366x768 laptop, with this patch networkmanagement window opens
in full screen height (respecting the panel)

Reviewers: #frameworks, ngraham

Reviewed By: ngraham

Subscribers: davidedmundson, acrouthamel, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D15406