Prevent having to double-tap some check boxes

Check boxes that do not have a default value set usually required
you to tap them twice to toggle them on the very first time. This
patch checks the return value from GetValue and sets a default of
0 if the variable does not already exist in the Data Manager.

Change-Id: I1ff26e756764ed2a00b10102f13a5bce0119771a

gui/checkbox.cpp

@@ -78,8 +78,13 @@ GUICheckbox::GUICheckbox(xml_node<>* node)
 		if (attr)
 			mVarName = attr->value();
 		attr = child->first_attribute("default");
-		if (attr)
+		if (attr) {
 			DataManager::SetValue(mVarName, attr->value());
+		} else {
+			int val;
+			if (DataManager::GetValue(mVarName, val) != 0)
+				DataManager::SetValue(mVarName, 0); // Prevents check boxes from having to be tapped twice the first time
+		}
 	}
 
 	mCheckW = mCheckH = 0;