Allow listbox to have a list of check boxes

For an example of how to implement a list of check boxes, see:

http://bit.ly/1Giacw2
https://gerrit.omnirom.org/#/c/14284/2/

Change-Id: I0752a084625f4a0ef8cc6d99597f2fb9aa2ab9e0
diff --git a/gui/objects.hpp b/gui/objects.hpp
index f8569d6..624db07 100644
--- a/gui/objects.hpp
+++ b/gui/objects.hpp
@@ -632,6 +632,7 @@
 protected:
 	struct ListData {
 		std::string displayName;
+		std::string variableName;
 		std::string variableValue;
 		unsigned int selected;
 		GUIAction* action;
@@ -643,6 +644,7 @@
 	std::string currentValue;
 	ImageResource* mIconSelected;
 	ImageResource* mIconUnselected;
+	bool isCheckList;
 };
 
 class GUIPartitionList : public GUIScrollList