Improve returning wxSYS_CARET_* metrics in wxMSW

The documentation stated that (a) wxSYS_CARET_{ON|OFF}_MSEC  settings
were not supported on MSW and (b) if a setting was not supported a
negative value was returned. However, when calling
wxSystemSettings::GetMetric(wxSYS_CARET_{ON|OFF}_MSEC), not only the
method asserted but 0 was returned instead of a negative value.

Fix this by using the value returned by ::GetCaretBlinkTime() for
wxSYS_CARET_{ON|OFF}_MSEC.

The documentation for wxSYS_CARET_TIMEOUT_MSEC does not list any
platform limitations; however, attempting to obtain this value resulted
in an assert and 0 returned. Fix this by returning -1. Returning -1 may
not the be the ideal solution, but there is no value reserved for the
setting not being supported and the caret on MSW usually blinks.

Closes https://github.com/wxWidgets/wxWidgets/pull/1285

src/msw/settings.cpp

@@ -253,7 +253,10 @@ static const int gs_metricsMap[] =
     SM_PENWINDOWS,
     SM_SHOWSOUNDS,
     SM_SWAPBUTTON,
-    -1   // wxSYS_DCLICK_MSEC - not available as system metric
+    -1,   // wxSYS_DCLICK_MSEC - not available as system metric
+    -1,   // wxSYS_CARET_ON_MSEC - not available as system metric
+    -1,   // wxSYS_CARET_OFF_MSEC - not available as system metric
+    -1    // wxSYS_CARET_TIMEOUT_MSEC - not available as system metric
 };
 
 // Get a system metric, e.g. scrollbar size
@@ -268,6 +271,25 @@ int wxSystemSettingsNative::GetMetric(wxSystemMetric index, wxWindow* WXUNUSED(w
         return ::GetDoubleClickTime();
     }
 
+    // return the caret blink time for both
+    // wxSYS_CARET_ON_MSEC and wxSYS_CARET_OFF_MSEC
+    if ( index == wxSYS_CARET_ON_MSEC || index == wxSYS_CARET_OFF_MSEC )
+    {
+        const UINT blinkTime = ::GetCaretBlinkTime();
+
+        if ( blinkTime == 0 ) // error
+        {
+            return -1;
+        }
+        
+        if ( blinkTime == INFINITE ) // caret does not blink
+        {
+            return 0;
+        }
+
+        return blinkTime;
+    }
+
     int indexMSW = gs_metricsMap[index];
     if ( indexMSW == -1 )
     {