Generate erase background events for native controls too under MSW.

Generate erase background events for native controls too under MSW.

Remove the test which prevented the generation of erase background events from
wxMSW::wxWindow::HandleEraseBkgnd(). Although it is true that native controls
mostly erase background on their own, there are exceptions: we must erase the
background of wxToolBar ourselves, for example. More importantly, there is no
reason to prevent the user code from defining wxEVT_ERASE_BACKGROUND handlers
for the native controls as this works just fine under MSW (although it doesn't
under GTK nor probably other ports...). So also add a test erase background
handler to the toolbar sample to verify that this does work.

Closes #11514.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62805 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775