Custom-added QCH files gets marked as auto-detected and removed in next session.

Steps to Reproduce:

1. Add an external QCH file via Tools->Help->Documentation tab.
2. Click OK/Apply.
3. Exit the Options panel and then open it up again. Note that the new QCH file is now marked as "(auto-detected)".
   1. At this point, assuming the F1 context help works for the QCH file in question, it opens up fine.
4. Quit QtCreator and restart. The QCH file is no longer listed, and context help doesn't work anymore.

What I haven't tested and may test in the future:

• Specifically which release did this bug started to happen.
• Whether or not it happens on equivalent Linux installs.
• Whether or not it also happens on the offline installs.
• Test this on a debug build of QtCreator.
• Current alpha/beta builds of QtCreator.

Other notes:

• Seems to be a recent error, since I would have likely noticed this if the bug were present in the 3.3.* releases (although I may have missed the later bugfix releases.)
• I've attached the QCH file I've extracted from downloading the Windows compile of Qbs for convenience as it's the smallest one I've tested, but it also affects CMake Sphinx-generated QCH docs and the ones from CppReference (not attached due to size) so I'll assume this isn't limited to these particular files.
• Deleting the QtCreator settings files does not help.