Constant filters

[robinp]

Now a top-bar search or a reference search doesn't have any fixed constraints, making it harder to do repeated search subject to same constraints.

There could be a combobox for additional constant query expression for top-bar. (Should this also affect ref results by default? Probably, so this would be an ultimate global filter).

And there could be an other combobox for the results filter - this can be used to quickly filter away certain file types, or test patterns.