Give completion popup a minimum size when editing an existing message

This was previously fixed to the size of the text area itself, which
could be small. We also have a bug where the text area *itself* is like
one character big, so this helps there too.

The popup is also given a margin of 0 so it is always positioned within
the bounds of the window.

BUG: 503483

(cherry picked from commit 142312d87d)

src/timeline/ChatBarComponent.qml

@@ -121,9 +121,11 @@ QQC2.TextArea {
 
     CompletionMenu {
         id: completionMenu
+        width: Math.max(350, root.width - 1)
         height: implicitHeight
         y: -height - 5
         z: 10
+        margins: 0
         connection: root.room.connection
         chatDocumentHandler: documentHandler
         Behavior on height {