Make inheritance rules consistent

[GPMueller]

Forwarding is described in the documentation. Currently, private include directories are made public for headeronly targets, but private flags stay private and are therefore never used.

This issue is related to issue #77.

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The complete set of inheritance rules should be:

• private flags and includes are
  • changed to public for headeronly targets
  • applied to target itself, unless headeronly
  • never applied to dependent targets
  • never forwarded
• public flags and directories are
  • applied to the target itself, unless headeronly
  • applied to all dependent targets, except headeronly
  • always forwarded
• interface flags are
  • never applied to the target itself
  • applied only to dependent shared libraries and executables
  • forwarded only by dependent static libraries