Erase toolbar background even if it doesn't contain any controls/spaces.

Erase toolbar background even if it doesn't contain any controls/spaces.

Minor correction to changes in r62971: we still need to hook WM_ERASEBKGND in
toolbar parent when painting the toolbar itself even when there are no dummy
separators in it (and so we don't need to paint them over). Without this, the
background of simple toolbars without controls was not erased correctly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62980 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775