Don't use themed border for the generic info bar.

Don't use themed border for the generic info bar.

Since the switch to deriving from wxControl instead of wxWindow, the info bar
gained a themed border which doesn't look nice for it, especially when it is
shown progressively, as happens under Windows.

Restore the old behaviour by overriding GetDefaultBorder() to return
wxBORDER_NONE for it. This doesn't look perfect neither however, we may want
to draw a separator line between it and the parent window ourselves as even
wxBORDER_SIMPLE looks too heavy for it (and there should be definitely no
border on its sides as it always touches the parent window edges).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62287 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775