Allow for different easing/transition types on leave/enter

[Robert-K]

I'd like to have a layout with different leave and enter transitions (different easing, transition type & duration).
Currently the only way to accomplish that with this plugin would be to add two GUITransitions, configured for enter and leave respectively.
The annoying part is triggering that enter/leave animation, as the enter GUITransition still plays it's leave animation and vice versa, so you have to differentiate them and cannot simply use the singleton and same layout ID.
A solution to this problem would be to allow setting enter/leave animations to NONE/DISABLED, but I believe there's a more elegant solution:
Instead of the GUITransition's Enter/Leave Animation properties, give a trigger property: ENTER/LEAVE/BOTH.
This way, adding transitions that act the same on leave/enter is still quite easy. Adding different transitions for LEAVE & ENTER is handled by adding two GUITransisitons set to LEAVE and ENTER, with full individual control.