Xmgr: hot keys

## Hot keys

When the pointer focus is on the canvas (where the graph is drawn), there are some shortcuts to activate several actions. They are:

- ^A - autoscale plot
- ^B - draw a box (using the current settings for boxes)
- ^D - delete an object (string, line, or box)
- ^N - move an object (string, line, or box)
- ^P - draw a line (using the current settings for lines)
- ^V - set the viewport with the mouse
- ^W - write a string (using the current settings for strings)
- ^X - exit (abruptly) xmgr.
- ^Z - enable zoom