Split GTK integration into a singleton

Telegram/SourceFiles/platform/linux/linux_gtk_file_dialog.h

@@ -0,0 +1,31 @@
+/*
+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
+
+#include "core/file_utilities.h"
+
+namespace Platform {
+namespace FileDialog {
+namespace Gtk {
+
+using Type = ::FileDialog::internal::Type;
+
+bool Supported();
+bool Use(Type type = Type::ReadFile);
+bool Get(
+		QPointer<QWidget> parent,
+		QStringList &files,
+		QByteArray &remoteContent,
+		const QString &caption,
+		const QString &filter,
+		Type type,
+		QString startFile);
+
+} // namespace Gtk
+} // namespace FileDialog
+} // namespace Platform