Now that WindowServer broadcasts the system theme using an anonymous
file, we need clients to pledge "recvfd" so they can receive it.
Some programs keep the "shared_buffer" pledge since it's still used for
a handful of things.
Whilst SystemMenu maintains a constant WindowServer connection, there
isn't always a Window active on that connection to listen for theme
changes - this causes the current theme in this process to fall out of
sync with the rest of the system. This fixes this problem by
re-requesting an UpdateSystemTheme message when the dialog is
instantiated.
