Remove selection help from SelectionCollection

Instead, make the only caller outside the selection module to use
selhelp.h directly.  This is not significantly messier, and allows
removing one use of gmx_unique_ptr from an installed header.

To actually remove that instance, do not install helptopicinterface.h.
This has the added benefit that now the whole onlinehelp module is free
from public headers, making the interface cleaner.  Adjust doxygen
documentation accordingly.

Part of #1454

Change-Id: Ifb557da86fb0d422650029812d30cc6766730fb9