selection: Support the file transfer portal

When the selection target is application/vnd.portal.files,
send files through the file transfer portal.

gtk/meson.build

@@ -659,6 +659,10 @@ if os_unix and tracker3_enabled
   gtk_unix_sources += 'gtksearchenginetracker3.c'
 endif
 
+if os_unix
+  gtk_unix_sources += 'filetransferportal.c'
+endif
+
 if os_unix
   gtk_sources += gtk_unix_sources
 endif