Move tg:// handlers to a separate module.

Also move joinGroupByHash and stickerSetBox from MainWidget.

Telegram/SourceFiles/core/local_url_handlers.h

@@ -0,0 +1,25 @@
+/*
+This file is part of Telegram Desktop,
+the official desktop application for the Telegram messaging service.
+
+For license and copyright information please follow this link:
+https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
+*/
+#pragma once
+
+namespace qthelp {
+class RegularExpressionMatch;
+} // namespace qthelp
+
+namespace Core {
+
+struct LocalUrlHandler {
+	QString expression;
+	Fn<bool(
+		const qthelp::RegularExpressionMatch &match,
+		const QVariant &context)> handler;
+};
+
+const std::vector<LocalUrlHandler> &LocalUrlHandlers();
+
+} // namespace Core