Optimize for repeated calls to setFilter.

We want filtering in quickopen to be as fast as possible. Thus
minimize the required conversions esp. get rid of the IndexedString
alltogether. Instead, we reuse (and implicitly share) the key value
of our sorting map now. This is now a QString which makes things
a tad bit slower at insert/remove yet pays off for non-ascii paths
and esp. during filtering as no conversions are required there anymore
then.

Benchmarks show that the add/remove test regresses about 50%, but
is still quite fast. Filtering and data() though got much faster which
is what I aimed at.