Improve diet multi combo in editor

[Robot8A]

Add support for values other than yes in multi combo.
Solves issue #496

[Robot8A]

This previous edit will allow values to be not only vegetarian, vegan... but also vegetarian=yes, vegetarian=only, ...
It splits the values with the = sign, if the key is split in two, there is a match and gets the second part as the value, otherwise still add =yes.

[Zverik]

Sorry, I struggle to find where this edit makes sense. Could you point me at a field that has an equals sign in its key?

[Robot8A]

On the iD tagging schema it does not. My idea was to change the keys on the preset.db so they somehow could store both the key and value (in this case with an equal sign).

So if in the iD tagging schema we would change

{
    "key": "diet:",
    "type": "multiCombo",
    "label": "Diet Types",
    "terms": [
        "gluten free",
        "fruitarian",
        "halal",
        "kosher",
        "lactose free",
        "meat",
        "pescatarian",
        "raw",
        "vegan",
        "vegetarian"
    ],
    "strings": {
        "options": {
            "vegetarian": "Vegetarian",
            "vegan": "Vegan",
            "halal": "Halal",
            "gluten_free": "Gluten-Free",
            "kosher": "Kosher",
            "lactose_free": "Lactose-Free",
            "pescetarian": "Pescetarian"
        }
    }
}

to

{
    "key": "diet:",
    "type": "multiCombo",
    "label": "Diet Types",
    "terms": [
        "gluten free",
        "fruitarian",
        "halal",
        "kosher",
        "lactose free",
        "meat",
        "pescatarian",
        "raw",
        "vegan",
        "vegetarian"
    ],
    "strings": {
        "options": {
            "vegetarian=only": "Vegetarian only",
            "vegetarian=yes": "Vegetarian friendly",
            "vegetarian=no": "Vegetarian no",
            ...
        }
    }
}

then it would work with my edit. But maybe it is not the way it should go, I am completely new to the code on this two repositories, so I may not be understanding the whole picture. Then maybe a new combo type should be discussed on the iD tagging repo.

[Zverik]

So this PR and suggestions sadly do not work, but I'll try to come up with another solution :) Thanks for having worked on this Héctor!