Remove content classes on close, fix transitionend listener & set default transform...

Remove content classes on close, fix transitionend listener & set default transform none for content

Now the content container has none transform by default so position fixed works as soon as an push off-canvas is closed.
Besides the transitionend listener is now fixed so it isn't bound to only the content or the element but to both of them.
The content classes specification is now part of the setup (this.contentClasses) for an improved readability.