Default Icon For Browse Pane

[jonwinton]

We have icons for "Tags", "Time" and "Person", but we don't have a default icon that just means "some sort of logic". This means a user has to pick between one of the three we include to have the logic displayed in the pane. Can we find a good default icon that any type of logic can use? The developer must choose the best icon to represent their chosen logic, but a fallback is necessary.

[amycheng]

closing in favor of #92