feat(overlay): add custom classes for backdrop

[kara]

This PR allows components to add a custom class to the overlay backdrop. This will allow us to use the backdrop as a click catcher in components like the menu and the select.

r: @jelbourn

[fxck]

#1438

[kara]

@fxck Can you elaborate? Your PR appears to have to do with overlay panes, not overlay backdrops. I assume @jelbourn will want to weigh in on the design for that one separately.

[fxck]

Hm, thought the backdrop was a child of the pane and thus the custom class on it could be used, but it's a child of the overlay.. so yeah. Ignore my comment.

[jelbourn]

LGTM

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