Improve handling of right-to-left support in wxStaticBox on wxMSW.

Back in #8101 I made fixes for wxStaticBox and right-to-left handling under Windows.

While this worked fine in wx2.8, the old patch has some unfortunate consequences on wx3:
* Since the box is always set to LTR, its children also inherit LTR
* Text was always right-aligned

This follow-up patch removes the RTL-specific code from wxStaticBox. Instead, the wxMemoryDC in wxStaticBox::OnPaint is made to inherit attributes from the wxPaintDC.

Tested on XP (both XP and classic theme), Windows 7 and 8.1

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76782 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775