[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.

Test Plan:
Before:

After:

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: abetts, ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10586

Differential Revision: https://phabricator.kde.org/D19670