[color-picker] Port configuration window to QQC2 and Kirigami.FormLayout
Summary: Port color-picker's configuration window to QQC2 and Kirigami.FormLayout & change the General category's icon so that it matches the applet's icon.
Everything still worked.
Reviewed By: VDG, ngraham
Differential Revision: https://phabricator.kde.org/D19789