This commit adds support for an additional `moc_extra_arguments` keyword.
It becomes especially handy, when `moc`-ed sources conditionally provide
`slots`, depending on compile time macros (i.e. defines).
This allows a CustomTarget to be indexed, and the resulting indexed
value (a CustomTargetIndex type), to be used as a source in other
targets. This will confer a dependency on the original target, but only
inserts the source file returning by index the original target's
outputs. This can allow a CustomTarget that creates both a header and a
code file to have it's outputs split, for example.
Fixes#1470