KRA/ORA: merged in a single plugin and added metadata support

- ORA/KRA: merged in a single plugin (KRA)
- ORA: removed standalone plugin
- KRA: use of logging category
- KRA: add metadata support by processing the embedded `documentinfo.xml`
- Improved signature lookup (see MR !17) and added test case

autotests/read/kra/src.kra.json

@@ -0,0 +1,19 @@
+[
+    {
+        "fileName" : "src.png",
+        "metadata" : [
+            {
+                "key" : "CreationDate",
+                "value" : "2016-01-27T10:40:16"
+            },
+            {
+                "key" : "ModificationDate",
+                "value" : "2016-01-27T10:41:28"
+            },
+            {
+                "key" : "Author" ,
+                "value" : "Boudewijn Rempt"
+            }
+        ]
+    }
+]