workspace: Only change opacity of minimized windows during transitions

Dragging a window preview in the overview is supposed to change the opacity of the dragged actor. This however fails for minimized windows, because Workspace::allocate() also changes the opacity of those. The allocation gets triggered by removing the window actor from the workspace when starting the drag. Avoid this by only changing the opacity during the overview transitions.

Since overview.animationInProgress would not include the transitions to or from the app grid, check for changes in the size of the workspace container instead.

Fixes #4292 (closed)