improve popup context menu to remove cruft

e.g.

• rename "Add Node..." to Add - can this be the first the group (below Copy/Paste)?
• remove "Update"
• consolidate "Remove" and "Delete" so there's just 1 item (though the key binding makes Remove nicer)
• zap the section above Export diagram (Team, Compare With, Replace With) etc
• move the Custom submenu up to its own section in the root popup menu (layout, show/hide grid)
• maybe move the Layout Diagram & Show Grid to a separe group?
• can we change the "Show Grid" menu item to be a radio button for show grid - or to change its text to Show Grid / HIde Grid?