Disable undo and selection-zoom buttons on graph when these are not applicable

[evertrol]

A minor but nice enhancement would be to disable icons & buttons that are not applicable. One obvious way is to grey-out the relevant icons & buttons (e.g., increase the transparency).

In this particular case, the undo button on a graph can be disabled when there is nothing to undo. Similarly, the zoom-to-selection button can be disabled when there is no data selected in the graph.