Improve editing messages with link previews.

Now preview state can be one of (allowed, cancelled, empty-in-edit).

In case of editing a message without preview we set the state to
empty-in-edit and it changes to allowed if the links in the message
are changed somehow.

That way we don't need to cancel the preview when editing a message
with a cancelled preview and at the same time adding a link to
a message that had no preview in the first place will add a preview.

Telegram/SourceFiles/chat_helpers/message_field.cpp

@@ -501,6 +501,11 @@ MessageLinksParser::MessageLinksParser(not_null<Ui::InputField*> field)
 	_field->installEventFilter(this);
 }
 
+void MessageLinksParser::parseNow() {
+	_timer.cancel();
+	parse();
+}
+
 bool MessageLinksParser::eventFilter(QObject *object, QEvent *event) {
 	if (object == _field) {
 		if (event->type() == QEvent::KeyPress) {