Define a way to dynamically add/remove nodes in plottr

[wpfff]

we need:

• programmatic way to insert/remove processing nodes at specified locations in the flowchart (a 'stack' of nodes?)
• a widget that allows editing the stack in the GUI
• specifying stack from config
• selecting things to add to the stack -- where does it look?
• must be plugin compatible -- user nodes must be found, loadable

check out: pyqtgraph examples for graphical flowchart, pyqtgraph example for library