Rework filtering UI

[SebastianZ]

In #220, @PolariTOON suggested to replace the drop-down list currently used to filter the tests by checkboxes for each specification plus some for stability options to improve the flexibility of filtering. In relation to that, the various CSS snapshot filters should only display the differences to the previous one.

I tend to agree on that, though at the same time I think the UI should't be cluttered with options. So my take on this would be to keep the drop-down for simple filtering but introduce an option "Custom...". Selecting that option displays a checkbox for each snapshot plus the ones for spec. stability as discussed in #239 and one to select all of them.
We could then also add an option to get the current absolute snapshot filtering back, i.e. all specifications added in the selected snapshot plus all the ones added in previous snapshots.

Sebastian

[LeaVerou]

I love the proposed UI!