*Wrap all startup notification calls in x11-only conditional blocks
*This is x11-specific code, so don't try to run it outside of x11

*put XKillClient and gdk_x11_screen_get_xscreen behind x11-only runtime checks

It is not possible to use the `list` pointer after it has been deleted,
so the "cleanup" this commit made lead to using freed memory if any
item actually got clean up.

This "cleanup" also don't seem meaningful to me, as all it does
otherwise is trade an assignation for a redundant test -- either of
which the compiler might happily optimize out.

This reverts commit 47426c90d10e9f738ecf89f35db94ca8deff55e0.

The timestamp is compared with g_file_info_get_attribute_uint64
in mate_desktop_item_get_file_status (const MateDesktopItem *item)

The timestamp is retrieved with g_file_info_get_attribute_uint64
in get_mtime (const char *filename)