Styles: Add terminal style class

[wpkelso]

• Deprecate Granite.STYLE_CLASS_TERMINAL
• Add undocumented terminal style class

[wpkelso]

Open question: Might this be better used as an internal style to a new TerminalView widget? Every time this is used in our repos, it's in the context of a ScrollBox with a TextView child.

[danirabbit]

Yeah I actually wonder if we should have this public facing at all. Because we mainly use it in messagedialog. The sideload and installer cases I think I'd be okay with them using undocumented API or just supplying their own styles. I'm not sure this style is actually useful outside of system apps since everyone else is expected to use MessageDialog

[wpkelso]

I'm happy with the style class being undocumented or internal. I think it's still worth it to abstract all of it into a widget, as even in the system apps and widgets the use cases for a terminal view are so similar that it should make the code cleaner. I've started another branch off this one to play around with that idea in #867