Don't post process events twice but return correct value from ProcessEvent().

Don't post process events twice but return correct value from ProcessEvent().

This is a further complication to the event handling code which aims to handle
correctly the case of wxScrollHelperEvtHandler which doesn't respect the
request to process events only in it but always passes it to the next handler
in the chain itself while still returning the correct value from
ProcessEvent() itself to avoid breaking code that relies on it, like the
background painting code in wxHtmlWindow.

This replaces the change of r64495 and does return true from DoTryChain() in
wxScrollHelperEvtHandler case but sets the "skipped" flag in the event itself
to indicate that it wasn't really processed and ProcessEvent() now checks it
after calling ProcessEventLocally() and returns the correct value
appropriately.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64510 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775