messagewebview: remove redundant default webview context menu

Gitlab: #58
Change-Id: I5dfd587a56289137b1e53de256482865de7f13df

src/mainview/components/MessageWebView.qml

@@ -261,6 +261,13 @@ Rectangle {
                 messageWebView.runJavaScript("displayNavbar(false);")
             }
         }
+
+        onContextMenuRequested: {
+            var needContextMenu = request.selectedText.length || request.isContentEditable
+            if (!needContextMenu)
+                request.accepted = true
+        }
+
         Component.onCompleted: {
             messageWebView.loadHtml(UtilsAdapter.qStringFromFile(
                                         ":/chatview.html"), ":/chatview.html")