[Suggestion] container of toolboxes medInria4

[mathildemerle]

In medInria 4, it could be interesting to be able to create a plugin which contain a combobox such as "ITK Basic Filters", without the need to have every toolbox of the combobox in the same plugin.

This could be linked to the plugin system changes #864 with the plugins labels/priority

This would allow to add external toolboxes to a combobox, for instance "ITK Basic Filters" or others. And it would be easier to manage plugins and toolboxes.