Reset wxSpinCtrl value to GetMin() if text string is invalid

Previously, wxSpinCtrl (using native control) and wxSpinCtrlDouble
(using the generic implementation) behaved differently in this case,
with the former changing its value but the latter keeping the last valid
value instead.

Make them behave the same by resetting the value in both cases and
document this behaviour.

tests/controls/spinctrltest.cpp

@@ -278,6 +278,7 @@ TEST_CASE_METHOD(SpinCtrlTestCase2, "SpinCtrl::Value", "[spinctrl]")
 
     m_spin->SetValue("");
     CHECK( m_spin->GetTextValue() == "" );
+    CHECK( m_spin->GetValue() == 0 );
 }
 
 TEST_CASE_METHOD(SpinCtrlTestCase2, "SpinCtrl::Base", "[spinctrl]")