Add prefix to css class to avoid conflicts

[yellow1912]

Right now some of the class names flowy uses seem to be quite generic (such as dragging, etc...). I think it would be nice to either:

1. Add a prefix flowy- in front of all the classes
2. Or, to avoid breaking changes with old codes, we can use multiple classes such as "flowy dragging". This allows us to specify the rules by using .flow.dragging for example while still keeping old code works.

[alyssaxuu]

That makes sense. Might be easier to just use a prefix like "flowy-", I agree that many of the class names currently being used are generic so it would lead to conflicts. I will look into it.