[FEATURE] "Multi-Select" for delay nodes

[jatinchowdhury18]

Is your feature request related to a problem? Please describe.
At the moment only one delay node can be "selected" at a time. However, there might be times when the user would want to "solo" multiple nodes, or changes parameter values for multiple nodes simultaneously.

Describe the solution you'd like
There should be a way to select multiple nodes, so that they can all be muted/soloed together, and have their parameters changed together as well.

Additional context
At the moment, the code is structured to expect exactly one selected node, so this will be something we'll have to deal with.