Add drag short-cut for Variable Getter/Setter

[Naros]

Description

When dragging a variable onto the graph canvas, it would be helpful to use short-cut keybindings to control how the drag is handled rather than forcing the user to select the Getter or Setter from the pop-up menu.

Implementation ideas

• Add a status message to indicate the short-cut keybindings, i.e. Ctrl=Setter, Shift=Getter.
• If the short-cut keybindings are pressed at drop-time, bypass the popup and place the Setter or Getter.
• If no short-cut keybinding is pressed at drop-time, present the user with the pop-up menu.