Script Generator: Load multiple scripts

As an INTER scientist, I would like to be able to select multiple scripts to load in the file selector and have them all be queued in the script generator.

The background is that INTER use their own script generator which has features currently missing from the IBEX script generator, and which does not currently integrate with Nicos. This feature would allow them to generate a script for each action from their script generator and easily load them into the script server.

A more ideal solution would be to help them integrate their script generator more directly with Nicos, however there have been past attempts at getting this to work which were not successful due to the script server acting in slightly idiosyncratic ways and automatically terminating connections. This ticket would serve as a "good enough" solution in the meantime.

### Acceptance Criteria
_What is the acceptance criteria?_
- [ ] When queueing a script in the script server perspective, there is an option to "queue multiple", which opens a file selector allowing the user to select a list of python files
- [ ] All scripts selected in this way are appended to the script server queue once the dialog is confirmed

### Extra Information
_Why it is needed?_

_Where required files/links are_

## How to Test
_verbose instructions for reviewer to test changes
(**Add before making a PR**)_