[cmake] Simplify header search for ROOT_GENERATE_DICTIONARY:

Enforce that headers *are* found, also by CMake, to ensure proper dependencies.
Add a backdoor for "you will not find the header": list those as GENHEADERS.
Add documentation, esp that headers should be passed to ROOT_GENERATE_DICTIONARY *as included*.
Use the directory (future: lib target) property for include directories.
Use these properties to pull in inc/ and v7/inc within ROOT_STANDARD_LIBRARY_PACKAGE, instead of hard-coded searches in ROOT_GENERATE_DICTIONARY.