Re-enabled write mode in KeePassLib

Set database as part of App (so, Singleton)
Changed main page view model
WIP and testing saving mechanism

ModernKeePassLib/project.json

@@ -5,6 +5,7 @@
     "NETStandard.Library": "2.0.0",
     "System.Net.Requests": "4.3.0",
     "System.Runtime.WindowsRuntime": "4.3.0",
+    "System.Xml.ReaderWriter": "4.3.0",
     "System.Xml.XmlSerializer": "4.3.0"
   },
   "frameworks": {