Add custom selections to 'multi-select' dropdown.

[Enviy]

As a user of the MITRE ATT&CK Navigator, it would be very helpful to add additional custom selections as available options under the 'multi-select' dropdown.

Currently there are three sections 'threat groups', 'software', and 'mitigations'. I have several layers that are configured to load by default but there are so many that they take up a lot of space in the layer tabs section. It would be very helpful to add the selections from those layers as entries under 'multi-select'.