🍒KF5: GlobalDrawer: Fix header with invisible content taking up space

That contentItem didn't refer to anything meaningful, and was probably an implementation oversight.

You should never bind parents' visibility to their children visibility, but here it is the only way to make some reasonable educated guess about parent size, so let's keep parent visible, and bind size instead.

Amends c3e00e58

(cherry picked from commit 24d929dd)