Layout change in CompositeKey user control

Button text is now settable
Opening database is placed in async task dispatcher to return control to the UI

ModernKeePass/Converters/EmptyStringToVisibilityConverter.cs

@@ -0,0 +1,20 @@
+using System;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Data;
+
+namespace ModernKeePass.Converters
+{
+    class EmptyStringToVisibilityConverter: IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, string language)
+        {
+            var text = value is string ? value.ToString() : string.Empty;
+            return string.IsNullOrEmpty(text) ? Visibility.Collapsed : Visibility.Visible;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, string language)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}