Touchscreen fixes

[a-tour-ist]

commit 1:
adding a table with color defined causes the string to start with tableoptions => formspec_elements[e] = nil => crash on L26

digiline_send("ts", {
    {
        command = "clear"
    },
    {
        command = "add",
        element = "table",
        color = "red"
    },
    {
        command = "modify",
        index = 1,
    }
})

commit 2:
setting real_coordinates once caused them to always be true in the formspec, no matter of the actual value.

mem = mem or {}
mem.real_coordinates = not mem.real_coordinates
digiline_send("ts", {
    {
        command = "clear" 
    },
    {
        command = "set", 
        real_coordinates = mem.real_coordinates
    },
    {
        command = "add",
        element = "button",
        name = "start",
        label = mem.real_coordinates and "real_coordinates=true" or "real_coordinates=false",  
        X = 0.3,  
        Y = 0.3, 
        W = 4,
        H = 4 
    },
})

commit 3:
trying to modify an item_grid caused the whole grid to disappear, except for the last item.

digiline_send("ts", {
    {
        command = "clear"
    },
    {
        command = "add",
        element = "item_grid",
        X = 0,
        Y = 0,
        W = 2,
        H = 2,
        name = "grid",
        spacing = 0,
        size = 1,
        interactable = true,
        items = {
            "default:dirt 99",
            "default:stone 42",
            "default:apple 13",
            "default:torch"
        },
        offset = 1,
    },
    {
        command = "modify",
        index= 1,
        X = 1
    },
})

[SwissalpS]

tested the three cases, LGTM.