HeaderBar: Use less clashing appmenu fallback icon

HeaderBar: Use less clashing appmenu fallback icon

process-stop-symbolic is unintuitive if represented as a stop sign as in
Adwaita, and completely ambiguous if represented as a cross like the
window close button in other icon themes.

Instead, use application-x-executable, which is already used elsewhere
as a fallback if no specific icon can be found for the application.

https://bugzilla.gnome.org/show_bug.cgi?id=784624