Add license

[tin2tin]

From my waveform experiments, this snippet will add license to the file data:

In UI:

            try:
                license = addon_data.freesound_list[addon_data.active_list_item].license
            except:
                license = "Unknown"
            row = col_list.row(align=True)
            split = row.split(factor=0.4, align=True)
            split.alignment = 'RIGHT'
            split.label(text="License")
            split = split.split(factor=0.6, align=True)
            split.alignment = 'LEFT'                
            split.label(text=license)```

And this in freesound.py:

```    license: StringProperty(
        name="License",
        description="The license of this sound",
        default="License"
    )```

[melMass]

What is the purpose? There is already https://github.com/iwkse/freesound/blob/7ac73f675187934f03b964b0b3e63e0a6951abc1/freesound.py#L232