[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:
{F6693452}

After:
{F6693453}

Everything still worked.

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

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