basic support for custom workspace icons

[fdev31]

This adds support for custom icons.

Since the settings gui for it is more complicated I left it for later.

Example usage (some of those icons don't look perfectly centered):

    "bar_workspace_icons": {
        "1": "",
        "2": "",
        "3": "",
        "4": "燎",
        "5": "蓼",
        "6" : "",
        "7" : "更",
        "8" : "󰭹",
        "9" : "亮",
        "10": "👽",
        "default": ""
    },

[fdev31]

Now implemented as part of #223