[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.

Test Plan:
Before:

After:

Everything still worked.

Reviewers: Plasma, VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D19789