GUI: Support styles in xml to reduce xml file size

Also allow sliders to have their own text label instead of
requiring a whole separate text object for the label in the xml.

Change-Id: I6e314efb4bb454d496555ff7e003d743063a1308
diff --git a/gui/progressbar.cpp b/gui/progressbar.cpp
index a49e0ab..a478a40 100644
--- a/gui/progressbar.cpp
+++ b/gui/progressbar.cpp
@@ -42,7 +42,7 @@
 		return;
 	}
 
-	child = node->first_node("resource");
+	child = FindNode(node, "resource");
 	if (child)
 	{
 		mEmptyBar = LoadAttrImage(child, "empty");
@@ -50,10 +50,10 @@
 	}
 
 	// Load the placement
-	LoadPlacement(node->first_node("placement"), &mRenderX, &mRenderY);
+	LoadPlacement(FindNode(node, "placement"), &mRenderX, &mRenderY);
 
 	// Load the data
-	child = node->first_node("data");
+	child = FindNode(node, "data");
 	if (child)
 	{
 		mMinValVar = LoadAttrString(child, "min");