PluginManager: test cleanup and unification.
UseCorrade.cmake: warn when using `0` instead of `nullptr`.

Available in GCC 4.7.
Utility: new macro CORRADE_ASSERT_UNREACHABLE().

Should be used instead of CORRADE_ASSERT(false).
Utility: doc++
Moved visibility macros into one common header.
Removed class diagrams from Doxygen documentation, minor other changes.

Consistency with other projects, especially Magnum.
Merge branch 'master' into compatibility
GCC 4.4 compatibility: __builtin_unreachable() is not yet available.