Add some benchmarks for bad performance when excluding large folders

When we open a project with a subfolder containing many files and
then chose to exclude that subfolder, we will get one
`fileRemovedFromSet` signal emission per contained file. Each one
of those will then do a lookup in the vector of project files,
which is potentially very slow.

Measurement on my machine:
```
RESULT : BenchIndexedString::bench_string_vector():"std::vector":
     285 msecs per iteration (total: 285, iterations: 1)
PASS   : BenchIndexedString::bench_string_vector(QVector)
RESULT : BenchIndexedString::bench_string_vector():"QVector":
     3 msecs per iteration (total: 3, iterations: 1)
```