single/dual cursor modes

Add the ability to enable either one of both T/Y cursors.

The cursor interfaces are given methods to set the individual cursor
visibilities. Cursor reset is moved to the plot interface, where it more
naturally resides. The plot menu gets simple widgets for enabling the two
cursors, and resetting their values.
3 jobs for single-dual-cursor in 5 minutes and 43 seconds (queued for 2 seconds)