[j.grpbox] rename -> HeinzGroupBox; don't inherit QGroupBox; don't require layout as c'tor arg; expose body() ()

Merging branch 'j.grpbox'  into 'main'.

See merge request !2063

Merging branch 'j.wui5'  into 'main'.

See merge request !2062

[j.gui4] Move code in GUI/View; expose "expand" bit w/o getter and setter; let GroupBoxCollapser take care of boilerplate set and connect statements ()

Merging branch 'j.gui4'  into 'main'.

See merge request !2059