Add support for G-S-D's media-keys extension

This fixes media keys handling on (but not limited to, probably):
* GNOME
* Cinnamon
* MATE
* Budgie
* Pantheon (elementaryOS)
* Unity

Telegram/SourceFiles/platform/linux/linux_gsd_media_keys.h

@@ -0,0 +1,36 @@
+/*
+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
+
+typedef struct _GDBusConnection GDBusConnection;
+
+namespace Platform {
+namespace internal {
+
+class GSDMediaKeys {
+public:
+	GSDMediaKeys();
+
+	GSDMediaKeys(const GSDMediaKeys &other) = delete;
+	GSDMediaKeys &operator=(const GSDMediaKeys &other) = delete;
+	GSDMediaKeys(GSDMediaKeys &&other) = delete;
+	GSDMediaKeys &operator=(GSDMediaKeys &&other) = delete;
+
+	~GSDMediaKeys();
+
+private:
+	GDBusConnection *_dbusConnection = nullptr;
+	QString _service;
+	QString _objectPath;
+	QString _interface;
+	uint _signalId = 0;
+	bool _grabbed = false;
+};
+
+} // namespace internal
+} // namespace Platform