move call for removeBackdrop() to backdrop()

[tlindig]

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.

[fat]

ah yeah i really like this change

[cvrebert]

@fat So, are you okay with merging this for 3.2? (I'm happy to do the rebasing / conflict-resolution.)

[fat]

@cvrebert yes please, thanks!