Preserve comment order

Some doxygen comments are not associated with any element translatable into C++, e.g. opening and closing groups: those are stand-alone. Accordingly, comments (and the elements they may refer to) must not be resorted when generating the output.

In addition, one-line comments must preserve empty lines between them: putting them together causes the interpretation to change, e.g. a @{ followed by a @brief always opens a group, but the brief is part of the group instead of the following element if there is no empty line between the two comments.