Add wxIntegerValidatorctor ctor taking minimum and maximum value

Using this ctor is more convenient than using the default ctor and then
calling SetMin() and SetMax().

Document the new ctor and add tests showing that minimum and maximum
values are actually respected.

Closes https://github.com/wxWidgets/wxWidgets/pull/2610

include/wx/valnum.h

@@ -347,6 +347,19 @@ public:
         this->SetMax(std::numeric_limits<ValueType>::max());
     }
 
+  // Ctor for an integer validator.
+  //
+  // Sets the range to the specified interval [min, max].
+  wxIntegerValidator(ValueType *value,
+                     ValueType min,
+                     ValueType max,
+                     int style = wxNUM_VAL_DEFAULT)
+      : Base(value, style)
+  {
+    this->SetMin(min);
+    this->SetMax(max);
+  }
+
     virtual wxObject *Clone() const wxOVERRIDE { return new wxIntegerValidator(*this); }
 
     virtual bool IsInRange(LongestValueType value) const wxOVERRIDE