add support for descriptions of sublibraries - shows category descriptions for http libraries in the symbol chooser

This merge request allows sublibraries to also display a description in the choose symbol dialog. The following print screens show the change.

Previously:

Afterward:

If no global description of the library is recorded, only the descriptions of the sub-libraries are displayed:

For the http library, the data can be supplied in the categories request as follows:

[
    {
        "id": "2",
        "name": "Chip Resistor",
        "description": "SMD chip resistors"
    },
    {
        "id": "3",
        "name": "MLCC Chip Capacitor",
        "description": "SMD chip multi layer ceramic capacitor"
    },
    {
        "id": "1",
        "name": "Power",
        "description": "Power symbols to create global power labels"
    }
]