add Group::clear(), do not clear _canvas member of Item when unparented (only the parent is changed)
Revert "add Group::clear(), do not clear _canvas member of Item when unparented (only the parent is changed)"

This reverts commit a4df65a56f83823aba27432685977f420458a213.
add Group::clear(), do not clear _canvas member of Item when unparented (only the parent is changed)
remove probably premature-or-no-longer-necessary optimization of measure line drawing