giscanner: parse block comments for members and fields

There was previously no mechanism for tagging enum members and struct fields with Since tags (or other, eg deprecation tags).

While the customary place to add Since tags for these symbols is inline in the parent symbol's documentation eg:

/**
 * Foo:
 *
 * @FOO_BAR: some bar. Since X.Y
 */

And variations on that theme, implementing parsing for that scheme would result in a pretty ambiguous grammar, especially if we also want support for multiple tags.

Instead, the solution implemented here is to allow providing documentation for individual members and fields through their own separate block, as is done for virtual functions already.

Inline comments are still used, with a lower precedence.

Fixes #348 (closed)