Support .titlebar class for headerbars

[cassidyjames]

It's a convention in elementary and AppCenter apps to use a .titlebar class on the headerbar to be able to reduce the padding and make it look like a normal titlebar while still packing in some buttons. elementary Terminal, for example, uses this for its search and font-size buttons.

Something like this would support that convention:

headerbar.titlebar {
	padding-top: 0;
	padding-bottom: 0;
}

headerbar.titlebar button {
	margin-top: 0;
	margin-bottom: 0; 
}

That takes elementary Terminal in Pop!_OS from this:

...to this:

...which is more how it's designed to look.

[cassidyjames]

Re-opening this since the previous fix broke other things and has been reverted.

[isantop]

Re-fixed in 9498c5f