Generate wxEVT_SPINCTRL of the correct wxSpinEvent type.

Handlers for this event expect to get wxSpinEvent but wxMSW and wxGTK1 sent an
object of base wxCommandEvent class which resulted in invalid memory access in
the handlers when using the derived class methods such as Veto().

See #16948.

docs/changes.txt

@@ -615,6 +615,7 @@ wxMSW:
 - Fix wxEVT_TREE_STATE_IMAGE_CLICK generation (Antal).
 - Fix wxDV_ROW_LINES in horizontally scrolled wxDataViewCtrl.
 - Fix RegisterHotKey() with negative IDs (troelsk).
+- Fix event object type for wxEVT_SPINCTRL events.
 
 wxOSX: