Toolbox context menu replace action should not be active when no workflow node is selected

[glopesdev]

The operator replace action is simply an operator insertion action with a modifier, and this is co-opted by the context menu to implement event handling when clicking the Replace context menu item.

However, from an interface point of view it doesn't make much sense to allow for the Replace action to be called using the menu when no node is selected, since the observed behavior in this case will be an insertion instead.