Template classes cannot have SWIFTEN_API annotations as with annotation code for instantiations of the template are expected to be in the DLL which cannot be guaranteed for any user type. With the complete implementation in available in the header it is not needed because 3rdParty Swiften users can instantiate an implementation as needed. This also conditionally includes SQLiteHistoryStorage.h conditionally in MemoryStorages.cpp, as otherwise the linker will expect an implementation of SQLiteHistoryStorage in the DLL. However, it is only built into the DLL if experimental features are turned on. Test-Information: Tested with experimental=on/off and swiften_dll=on/off. Change-Id: Ieec85675c167ec34cffd4745ac854a5949fb2037