Add value for bottom padding on custom slide

[Mauker1]

When we're creating a custom slide, we need to know the padding that we should add at the bottom, and I couldn't find this value anywhere in the docs, so I had to dig into the code until I found this: android:paddingBottom="164dp".

My suggestion is to create a value on the dimens folder for that particular padding, like customSlideBottomPadding, since it's a must have on custom slides.

You could also add a padding for a "buttonless" custom slide, so the user won't have a padding that is higher than necessary.