FindDoxygen: Improve `doxygen_add_docs`

• doxygen_add_docs now accepts targets as documentation input. And for this case would append its sources to the DOXYGEN_INPUT list. It is also possible to filter what sources should count using a list of regular expressions which are applied to all or individual targets;
• When DOXYGEN_INPUT consists of files only, it is possible to use add_custom_output instead of add_custom_target. so documentation gets rebuild only when something has really changed;
• There is an additional option ALL could be set to make added target to be executed by default build.

Topic-rename: FindDoxygen-improve-doxygen_add_docs