lib/pf4: Use alert font style for alert titles
lib/pf4: Use alert font style for alert titles

Fixes #15639

PF4 uses H4 for titles. However, H4 is often set with different font family and
size in other frameworks; notably, but not limited to PatternFly 3.

martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,garrett/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,garrett/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,garrett/cockpit,mvollmer/cockpit