Sidebar: Keep headers minimum height on two columns

The implicitHeader binding from sizeHelper is getting overriden
by Kirigami.AbstractApplicationHeader (line 76), which recalculates
a preferred height using the header's childrens implicit heights.

What's really keeping the header height at the desired value are the
back and hamburger buttons, so let's keep that size also when showing
two columns

BUG: 448889
FIXED-IN: 5.24


(cherry picked from commit 3cec4ed9)