Add the option to save extra data in the datatype folders

[marcelzwiers]

Is your feature request related to a problem? Please describe.
Users may want to save data that is not (yet) part of the BIDS standard within a datatype folder (e.g. anat).

Describe the solution you'd like
Now the bidseditor only provides extra_data as a datatype for saving non-BIDS data. This is rather limited

Describe alternatives you've considered
a bidseditor option for users to add key-value pairs manually