infobox manager: keep infoboxes in order of insertion

Collections.binarySearch() does not guarantee which element is found if
there are multiple that compare equal, leaving the order of infoboxes
not necessarily in insertion order. This was an unintended side effect of
ba9ffb1d60 and
406c2bc7db.

runelite-client/src/test/java/net/runelite/client/ui/overlay/infobox/InfoBoxManagerTest.java

@@ -114,5 +114,8 @@ public class InfoBoxManagerTest
 		infoBoxManager.addInfoBox(new TestInfobox(InfoBoxPriority.MED, "three"));
 
 		assertEquals(3, infoBoxManager.getInfoBoxes().size());
+		assertEquals("one", infoBoxManager.getInfoBoxes().get(0).getText());
+		assertEquals("two", infoBoxManager.getInfoBoxes().get(1).getText());
+		assertEquals("three", infoBoxManager.getInfoBoxes().get(2).getText());
 	}
 }