File actions menu: Allow user to apply action on multiple files at once

[StefanRichterHuber]

Describe why it is important and where it will be useful

Currently one can apply a file menu action only for one single file. It would be useful for the user to select multiple files and apply the file menu action on all at once.

Describe your proposed solution

To not change the protocol, trigger the configured action handler for each file in parallel.

If additions to the protocol are considered, add something like an optional multiselectActionHandler to the registration of a File Actions Menu, which is used when multiple files are selected and which receives a list of files at once. This would allow for advanced use cases like merging multiple files.

Describe alternatives you've considered, if relevant

No response

Additional context

No response

[bigcat88]

This PR implements it: #284

Now it is possible to process in one endpoint:

1. one file
2. multiple files
3. optionally ask for redirect to the ExApp TopMenu page(UI) with fileID's

If for ExApp there is need to process files in parallel, ExApp can handle on it is on it's own by creating threads/spawning subprocesses/etc..