* Add an undo buffer barrier when the skein async checkout operation begins
* Add a delegate handler for skein checkout operation completion
* Make sure auto checkout is only active in UEFN and/or if skein is enabled
* Add UI control for turning on and off Auto-checkout/Auto-undo in Source control settings
#rb stuart.hill, brooke.hubert, zach.rammell
[FYI] wouter.burgers, manuel.lang
#preflight 637b5227f514e1ded97739a4
[CL 23220340 by marco anastasi in ue5-main branch]