qt: regression: greedy expansion in center control bar

Expansion theoretically can not work in the center layout when left and right layouts are visible. It can only work when left and right become invisible. The reason is, a greedy control can easily occupy all the space and make non-expanding controls in other layouts invisible.

Expected behavior: controls must use their minimum size in the central layout, that is, disable expansion in center layout as long as left or right layout is visible.

Regression since !3385 (merged).