C and C++, alias fixes and improvements

[jakobandersen]

Feature or Bugfix

• Feature
• Bugfix

Purpose

• Fix a couple of issues in the c:alias directive.
• Add the maxdepth option to cpp:alias similar to the one in c:alias.
• Add a noroot option to c:alias and cpp:alias to enable insertion of all nested declarations of a symbol, without the symbol it self, e.g., for inserting a synopsis of a class.

Relates

Addresses the basic part of #8213.