Ensure initial value of generic wxSpinCtrl is always valid

Under MSW creating a wxSpinCtrl with a range of, say, 1..10 and the
default initial value of 0 sets its initial value to 1 (i.e. the closest
valid value) as expected, but the generic version still set it to the
invalid value of 0, which was unexpected, inconsistent and not useful.

Fix the generic version to follow MSW behaviour now and add a test
checking for this.

include/wx/generic/spinctlg.h

@@ -145,6 +145,9 @@ protected:
     // check if the value is in range
     bool InRange(double n) const { return (n >= m_min) && (n <= m_max); }
 
+    // adjust the value to fit the range and snap it to ticks if necessary
+    double AdjustAndSnap(double value) const;
+
     // ensure that the value is in range wrapping it round if necessary
     double AdjustToFitInRange(double value) const;