Boxes: give a way to omit the icon

[damithc]

In some cases the user might want to omit the icon of a box.

Perhaps we can allow that in some way e.g., icon=""?

[tlylt]

Should the space occupied by the icon be retained (or removed, i.e. the paragraph should be aligned further left) if the icon is removed?

[damithc]

Should the space occupied by the icon be retained (or removed, i.e. the paragraph should be aligned further left) if the icon is removed?

Retaining the space might give a more consistent look with other boxes (and also, easier to implement) -- not sure if the empty space make it look 'odd' though. I'm open to the other option too. We can try to easier option first and see how it looks?