Add wxSpinCtrl::SetBase() to allow entering hexadecimal numbers.

Add a generic SetBase() API even though right now only bases 10 and 16 are
supported as we might support other ones (e.g. 8?) in the future. Implement it
for MSW, GTK and generic versions.

Add controls allowing to test this feature to the widgets sample.

Add "base" property support to the XRC handler for wxSpinCtrl, document it and
test it in the xrc sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72414 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/overviews/xrc_format.h

@@ -1495,7 +1495,12 @@ HTML markup. Note that the markup has to be escaped:
 
 @subsubsection xrc_wxspinctrl wxSpinCtrl
 
-wxSpinCtrl supports the properties as @ref xrc_wxspinbutton.
+wxSpinCtrl supports the same properties as @ref xrc_wxspinbutton and, since
+wxWidgets 2.9.5, another one:
+@beginTable
+@row3col{base, integer,
+    Numeric base, currently can be only 10 or 16 (default: 10).}
+@endTable
 
 
 @subsubsection xrc_wxsplitterwindow wxSplitterWindow