Add gio launch command to execute desktop file

This command will try to execute a desktop file, before that
it will load the input as a keyfile for checking its existence
and its validity (as a keyfile).
File arguments are allowed after the desktop file.

Closes #54

Signed-off-by: Frederic Martinsons <frederic.martinsons@sigfox.com>

gio/meson.build

@@ -908,6 +908,7 @@ gio_tool_sources = [
   'gio-tool-cat.c',
   'gio-tool-copy.c',
   'gio-tool-info.c',
+  'gio-tool-launch.c',
   'gio-tool-list.c',
   'gio-tool-mime.c',
   'gio-tool-mkdir.c',