Reuse existing elements of PathFilter::m_items

This elimination of memory reallocation, redundant ProjectFile
construction and destruction, dramatically speeds up
ProjectFileDataProvider::reset() when a huge project is open.

Consider WebKit project with almost 320 thousand files in total and 13
of these files open: the average time spent in
ProjectFileDataProvider::reset() went from 80 ms to 7 ms (starting from
the second call to this function, because memory is still allocated and
all ProjectFile objects are still created in the first call to it).

BenchQuickOpen::benchProjectFileFilter_reset() runs 8 times faster with
this change.