Add XDG Desktop Portal based file dialog implementation from Qt

This allows to use portal dialogs more flexibly (e.g. fallback mechanism)
This also allows to have any changes we want for portal dialogs without patchig Qt

No more need to override QT_QPA_PLATFORM to use portal dialogs

Telegram/SourceFiles/platform/linux/file_utilities_linux.cpp

@@ -10,6 +10,10 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 #include "platform/linux/linux_gtk_integration.h"
 #include "platform/linux/specific_linux.h"
 
+#ifndef DESKTOP_APP_DISABLE_DBUS_INTEGRATION
+#include "platform/linux/linux_xdp_file_dialog.h"
+#endif // !DESKTOP_APP_DISABLE_DBUS_INTEGRATION
+
 #include <QtGui/QDesktopServices>
 
 extern "C" {
@@ -77,6 +81,18 @@ bool Get(
 	if (parent) {
 		parent = parent->window();
 	}
+#ifndef DESKTOP_APP_DISABLE_DBUS_INTEGRATION
+	if (XDP::Use(type)) {
+		return XDP::Get(
+			parent,
+			files,
+			remoteContent,
+			caption,
+			filter,
+			type,
+			startFile);
+	}
+#endif // !DESKTOP_APP_DISABLE_DBUS_INTEGRATION
 	if (const auto integration = GtkIntegration::Instance()) {
 		if (integration->fileDialogSupported()
 			&& integration->useFileDialog(type)) {