[appmenu] Port configuration window to QQC2 and Kirigami.FormLayout
Summary: This patch ports global menu's configuration window to QQC2 and Kirigami.FormLayout. It also changed the category icon to be consistent with other applets.
Reviewed By: VDG, ngraham
Subscribers: abetts, ngraham, plasma-devel
Maniphest Tasks: T10586
Differential Revision: https://phabricator.kde.org/D19670