Add .keycap class to conflict dialog

[cassidyjames]

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I don't think it's necessary in the listing (it would probably just be super noisy?), but we should probably use the .keycap class to key names in the conflict dialog. It can be confusing when a user has + as one of the keys, and since the stylesheet has support for it, I feel like we should use it.

[danirabbit]

Yeah I was thinking about that. One downside is we'd have to build the dialog manually instead of using Granite.MessageDialog since the primary text is just a single Gtk.Label. Another potential problem though is translations. I'm not sure if Pango accepts the style attribute or not