Improvements to cross section viewer

[ericdill]

@klauer has requested some improvements to the cross section widget.

1. pass in x and y motor positions
   1. pass in two vectors
   2. pass in the extent (opposite corners)
   3. pass in all of the motor positions

In terms of an MVP, I'll implement vectors and extent.

1. Add a button to select a rectangular roi that will emit a qt signal and will execute registered callbacks.

[ericdill]

@klauer also says that it would be really cool to have this done by the next cycle, but is OK if it is not done until halfway through the next cycle

[tacaswell]

Have a look at the updated rectangle selectors in 1.5rc1, they now have handles and can be resized interactively.

[ericdill]

In-person conversation with @klauer

A few possible uses:

• use this in place of the live updating hxnfly widget
• use this in the LiveRaster callback

To get additional functionality, we can use keypresses to do things like 'select a ROI'