textview: Include gutter while computing child allocations

The width of the left gutter and the height of the top gutter are now used while computing the child allocations for e.g. anchors, otherwise - if such a gutter is present - the widget would be at the wrong position.

Closes #5016 (closed)

(I changed the layout from LTR to RTL using the inspector and it seemed to work, but I'm not sure whether I really checked that it works right with RTL)