Background image should be undoable

[noahsark769]

When the background image is moved or resized, it should be undoable so you can move it back to where it was.

This would be a little tricky. Right now background image is part of the main frame canvas. To have it be part of the undo system we would have to move it into the document.

Though if it was in the document you could save it with the file. So you would have the background image placement preserved with every show. Food for thought.

[rmpowell77]

Relabeling this a feature request. Not really a bug, as it wasn't intended to operate that way.

[rmpowell77]

This should be easier than originally thought. The CalChartDoc object is what accept the commands. If we move the background image from outside the frame to inside the Doc, then we can have the commands be undoable.

Though we would have to be careful and not mark the doc as modified. This would mean a different base class for the command.

[rmpowell77]

This is addressed in #186 and #175.