Added command to remove unknown inputs/outputs/variables.
Fixed appearance of Event node variables.
Added labels to inputs/outputs/variables.

This is the human-readable text that appears in the editor.  Changing the
label doesn't break anything the way changing a name does.
Renamed BaseNode 'name' to 'label' for consistency with input/output/variable 'name' vs 'label'.
Fixed drawing issues with ConnectionItem.
Allow connections to/from the root node.
Added right-click menus to input/output items and connection items.

input/output context menu has a command to display the ConnectionsDialog
which shows the connections for the given input/output.

connection context menu has a command to remove the connection.
NodeConnectionsList shows 2 columns, "From" and "To"

Used to show Output/Target/Input, but now connections from input->input and
output->output are allowed.
Allow editing input/output labels of the root-node.
Added commands to context menus.