mouse: shrink split row into two rows
The newly introduced split row gets its minimum with from the minimum widths of their children, most importantly the labels.
The mouse and touchpad panel has therefore a high minimum width exceeding the width-request of 360px of the panel stack, introducing (a) jumps in width when switching panels, and (b) does not fit small screen such as those found on phones.
Letting the CcSplitRow shrink from two columns into two rows using GtkFlowBoxes ensures a mobile compatible panel, and still follows the recent mock-ups for larger widths.