Improve performance for large projects

Performance can (probably) be greatly improved for large projects. Do this with e.g. Git or the Linux Kernel.

This issue is largely limited to IFileStatusCache.MarkDirty and many consecutive calls to IFileStatusCache.RefreshItem without any of the items existing (the first project load).

This issue is solved by "detecting" a full refresh is done and then changing the depth from Files to Children.