Leave ownership of the native window to the user code as it may want to reuse
it for some other purpose and provide an explicit Disown() function that can
be called if the user really wants wxWidgets to take ownership of the native
window.
In particular, this avoids problems when using ARC under OS X which resulted
in a double "release" before.
Define BS_SPLITBUTTON ourselves if it's not defined in the headers to at
least create the control even then. Skip the code handling BCN_DROPDOWN as
it's too much trouble to make it compile with the old headers however.
This should fix the build with VC8 currently failing on buildbot.
