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

src/timeline/ChatBarComponent.qml

@@ -104,11 +104,13 @@ QQC2.Control {
 
                     CompletionMenu {
                         id: completionMenu
+                        width: Math.max(350, root.width - 1)
                         height: implicitHeight
                         y: -height - 5
                         z: 10
                         connection: root.Message.room.connection
                         chatDocumentHandler: documentHandler
+                        margins: 0
                         Behavior on height {
                             NumberAnimation {
                                 property: "height"