Revert "Try out a split window approach for NERDTree"

While the drawer approach is great, I loose a lot of the NERDTree
functionality I love like custom functions and easy toggling. Taking up
on the lazy nertree-tabs switching work that @StanAngeloff did, I am
able to enjoy NERDTree again.

Thanks, Stani :)

This reverts commit 61ea309685403406195a3b3c3170044879e61c01.

Conflicts:
	.vimrc