#1096 Inserting a plot using canvas context shortcut generates bug

[Hal Crichton-Standish]

When hovering the mouse over a blank space on the canvas (as suggested in the minsky manual in the canvas background section) and pressing '@' on the keyboard, the system returns the error message 'invalid command name "minsky.plots.nextPlotID" '. All other keyboard functions (e.g. +-/^ etc) work as intended. It might be that the code for inserting a Plot using this shortcut is missing or not working.

[High Performance Coder]

This one got missed in the 1.x->2.x refactor. Ooops!