gtk3, gtk4: unscrew libhandy's height

Looks like libhandy requires the style to set a minimum height
for titlebars; themes that fail to do so will get broken apps.
I picked 46px because that's what Adwaita uses, and I fear that
deviating from that will cause apps to potentially break.

BUG: 430081


(cherry picked from commit 9bd3044d)