helm-projectile.el (helm-projectile-define-key): performance optimizatio...

Hi Bozhidar,

I just optimized the macro `helm-source-projectile-projects` for the sake of performance. This is because "list" in Emacs Lisp is singly-linked list, frequently adding to the end of a list using `add-to-list` can be slow. For this reason, it's better to add to the front of a list, and just do a one-time reverse after adding all the items.