This adds a double-click speed slider control to the Mouse Settings
panel, and value labels for both the movement speed and double-click
speed sliders.
To allow for updating and persisting the configured double-click
speed through the WindowServer, two IPC calls - `SetDoubleClickSpeed`
and `GetDoubleClickSpeed` - have been added.
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.
