C++, alias directive, add recursiveness

[jakobandersen]

The alias directive can insert a declaration or a function overload set, with a link to the actual declaration. As discussed in #8141 for the C domain, it can be quite useful to insert nested declarations as well, e.g., to make a synopsis of a class.

• There should be a maxdepth option, as the one in the c:alias:. (added in v3.5)
• Perhaps there should be filtering options to include/exclude certain types of declarations. E.g., only list nested type/classes/functions.
• To make the previous point more usable, there should probably be an "only-nested" option. (added in v3.5, as a noroot option)
• Some of the options for the autodoc directives might be interesting to adapt?