Autogen: User defined dependency file filters for AUTOMOC

Sebastian Holtermannrequested to merge
sebholt/cmake:autogen_depends into master

These patches add the variable CMAKE_AUTOMOC_DEPEND_FILTERS and the target property AUTOMOC_DEPEND_FILTERS.

AUTOMOC_DEPEND_FILTERS are used to extract additional dependency files for moc from the content of a source file.

Documentation and test are included.

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