Replacement for QWS_DBLCLICK_DISTANCE

Add QT_DBL_CLICK_DIST to replace QWS_DBLCLICK_DISTANCE for controlling
the distance for detecting double clicks, which can be very useful on
embedded devices

[ChangeLog][QtGui] environment variable QT_DBL_CLICK_DIST 
customizes the amount of movement allowed when detecting a double click.

Task-number: QTBUG-40841
Change-Id: I0a7534ad6cd6387d127eb49021a92d414d45670e
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/gui/kernel/qplatformtheme.cpp

@@ -508,7 +508,11 @@ QVariant QPlatformTheme::defaultThemeHint(ThemeHint hint)
     case MousePressAndHoldInterval:
         return QVariant(800);
     case MouseDoubleClickDistance:
-        return QVariant(5);
+        {
+            bool ok = false;
+            int dist = qgetenv("QT_DBL_CLICK_DIST").toInt(&ok);
+            return QVariant(ok ? dist : 5);
+        }
     }
     return QVariant();
 }