Further simplificiation of the NSSlider/wxSlider interface such that the

Further simplificiation of the NSSlider/wxSlider interface such that the
various responder methods don't change the controls action but instead send
all actions through the normal wxCocoaNSControl action handling mechanism.
To determine what caused the action we save the message selector in a global
variable (safe since Cocoa does single-threaded event handling).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@48078 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775