adapt empty-view layout to overall style

reusing corner radius of the bubbles
and centering as info-messages
(the empty-view is sort of an info message as well).

this is also roughly in sync with the layout on desktop and android.

deltachat-ios/View/EmptyStateLabel.swift

@@ -7,8 +7,9 @@ class EmptyStateLabel: PaddingTextView {
         super.init()
         backgroundColor = DcColors.systemMessageBackgroundColor
         label.textColor = DcColors.systemMessageFontColor
-        layer.cornerRadius = 5
+        layer.cornerRadius = 16
         label.clipsToBounds = true
+        label.textAlignment = .center
         paddingTop = 15
         paddingBottom = 15
         paddingLeading = 15