We didn't add buttons for certain window types or states when the
window was created, but when a window with a button changed its
state to where we would not have created the button, we didn't
remove the existing button.
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.
Window icons in Taskbar were previously received in WM events with
shbuf ID's. Now that Gfx::ShareableBitmap is backed by anonymous files,
we can easily switch to using those.