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

interface/wx/settings.h

@@ -197,7 +197,7 @@ enum wxSystemMetric
         Time, in milliseconds, for how long a blinking caret should
         stay visible during a single blink cycle before it disappears.
         If this value is negative, the platform does not support the
-        user setting.  Implemented only on GTK+ and MacOS X.
+        user setting.
 
         @since 3.1.1
     */
@@ -208,8 +208,7 @@ enum wxSystemMetric
         stay invisible during a single blink cycle before it reappears.
         If this value is zero, carets should be visible all the time
         instead of blinking.  If the value is negative, the platform
-        does not support the user setting.  Implemented only on GTK+
-        and MacOS X.
+        does not support the user setting.
 
         @since 3.1.1
     */