views: task metadata filtering

[oliver-sanders]

We have the ability to filter tasks by name, it may also be useful to allow filtering by arbitrary task metadata fields.

See also the GScan equivalent: #581

A vague example:

[runtime]
    [[a, b, c]]
        [[[meta]]]
            model = a
            system = x
    [[d, e, f]]
        [[[meta]]]
            model = b
            system = x

It might also be useful to be able to filter by job properties e.g. platform.

Pull requests welcome!