sal_Int16 as the underlying type of WindowBorderStyle

because g++ 4.7.2 generates [-Werror=type-limits] as follows:
> [build CXX] vcl/source/window/settings.cxx
> [build CXX] vcl/source/window/paint.cxx
> [build CXX] vcl/source/window/resource.cxx
> [build CXX] vcl/source/window/accel.cxx
> [build CXX] vcl/source/window/accmgr.cxx
> [build CXX] vcl/source/window/brdwin.cxx
> [build CXX] vcl/source/window/accessibility.cxx
> [build CXX] vcl/source/window/legacyaccessibility.cxx
> [build CXX] vcl/source/window/clipping.cxx
> [build CXX] vcl/source/window/stacking.cxx
> [build CXX] vcl/source/window/debug.cxx
> [build CXX] vcl/source/window/globalization.cxx
> [build CXX] vcl/source/window/btndlg.cxx
> [build CXX] vcl/source/window/builder.cxx
> [build CXX] vcl/source/window/cmdevt.cxx
> [build CXX] vcl/source/window/cursor.cxx
> [build CXX] vcl/source/window/debugevent.cxx
> [build CXX] vcl/source/window/decoview.cxx
> In file included from /home/tabe/core/include/rsc/rsc-vcl-shared-types.hxx:24:0,
>                  from /home/tabe/core/include/vcl/keycodes.hxx:23,
>                  from /home/tabe/core/include/vcl/keycod.hxx:26,
>                  from /home/tabe/core/vcl/inc/svdata.hxx:35,
>                  from /home/tabe/core/vcl/source/window/brdwin.cxx:21:
> /home/tabe/core/include/o3tl/typed_flags_set.hxx: In instantiation of 'typename o3tl::typed_flags<T>::Wrap operator&(E, E) [with E = WindowBorderStyle; typename o3tl::typed_flags<T>::Wrap = o3tl::is_typed_flags<WindowBorderStyle, 12339>::Wrap]':
> /home/tabe/core/vcl/source/window/brdwin.cxx:1027:44:   required from here
> /home/tabe/core/include/o3tl/typed_flags_set.hxx:105:5: error: comparison is always true due to limited range of data type [-Werror=type-limits]
> /home/tabe/core/include/o3tl/typed_flags_set.hxx:106:5: error: comparison is always true due to limited range of data type [-Werror=type-limits]
> cc1plus: all warnings being treated as errors
> make[1]: *** [/home/tabe/build/workdir/CxxObject/vcl/source/window/brdwin.o] Error 1
> make[1]: *** Waiting for unfinished jobs....
> make: *** [vcl.all] Error 2
> tabe@thunk:~/build$

Cf. a6b01d01f77f84517d267bdfe31de91b9050a70c

Change-Id: Ic596eaf886d9aebb8a5b8636b5b90d5935aeadaf