Undo stack and visual properties improvements

The multi-widget context menu allows setting visual properties ("Show") for a
selection. Unlike the previous "toggle" behaviour it is now possible to (for
example) hide all operations with one menu action.

Also fixed:
* applying the properties dialog without changes added a few undo commands to the stack
* the "Delete" shortcut is renamed to "Delete Diagram" because that is what it does
* mixed capitalization cmd/Cmd, is now always "Cmd"
* changes to the following widget options are now tracked in the undo stack:
** visual properties
** use fill color
** line width
** font