Revert "Quick Open: QVector => std::vector of project files"

This reverts commit 73720389.

This allows us to leverage the Q_MOVABLE_TYPE optimization for
the ProjectFile, which is esp. handy when we react to individual
file removal after excluding a large subfolder in a project.

Previously:
```
PASS   : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet(1000)
RESULT : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet():"1000":
     6 msecs per iteration (total: 6, iterations: 1)
PASS   : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet(10000)
RESULT : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet():"10000":
     497 msecs per iteration (total: 497, iterations: 1)
```

Now:
```
PASS   : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet(1000)
RESULT : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet():"1000":
     3 msecs per iteration (total: 3, iterations: 1)
PASS   : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet(10000)
RESULT : BenchQuickOpen::benchProjectFileFilter_fileRemovedFromSet():"10000":
     281 msecs per iteration (total: 281, iterations: 1)
```