Workaround for problems linking as library.

Compilers (especially visual studio) can't be forced to link object
files which content isn't referenced, so continue the ugly tradition
to specify all widget factories in the same place.
Deprecate TBAddon and made TBImage optional by define.

Making features optional by linking is messier than using defines.
Use tb_config.h for all optional core stuff instead.
Removed special compiler flags for linking all object files.

Should not be needed anymore.
Mention cmake in build instructions.