Use GetWidgets() in the widgets samples instead of GetWidget2().

Use GetWidgets() in the widgets samples instead of GetWidget2().

Add a function which can be overridden to return an arbitrary number of
widgets instead of having just GetWidget() and GetWidget2(): spin control page
already uses 3 widgets (and defines GetWidget3() which is never called) and we
could have even more in the future. Just use a generic solution which will
always work.

The practical consequence of this is that the "Enable/Disable" menu item now
also enables and disables the wxSpinCtrlDouble in the spin page, see #12045.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64301 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775