Add wxSpinCtrl::SetIncrement() and implement it for all ports

SetIncrement() was already available in wxSpinCtrlDouble and GTK version
of wxSpinCtrl, now implement support for it in wxMSW and wxOSX as well.

In fact, in wxMSW, implement it at wxSpinButton level, so that both this
class and wxSpinCtrl inheriting from it (in wxMSW only) support setting
custom increment now.

Also add support for it to XRC, show it in the sample and add a unit
test verifying that it works.

Closes #2597.

docs/doxygen/overviews/xrc_format.h

@@ -2057,6 +2057,8 @@ exactly one non-toplevel window as its child.
     Minimum allowed value (default: 0).}
 @row3col{max, integer,
     Maximum allowed value (default: 100).}
+@row3col{inc, integer,
+    Increment (default: 1). Available since wxWidgets 3.1.6.}
 @endTable