move call for removeBackdrop() to backdrop()

function `backdrop` has a part, where the backdrop fade out is started. But the function `removeBackdrop()` was then called in 'hideModal'. That is not the optimal place for this. 

It makes more difficult to extend to `modal` class, because it must be override more functions than it ought to be.

For instance I would like to implements a different handling for backdrop. With the change of this pull request, I only have to overwrite the `backdorp` function.