[lib] Improve macros for library index entries

[jensmaurer]

• Make a macro so that every mention of a library header is auto-indexed, with a bold page number for the header synopsis.
• Make a macro to replace \indexlibrary{\idxcode{blah}}, e.g. \indexlibraryglobal{blah}. This complements \indexlibrarymember{member]{class}.
• Make a macro so that an index entry may be performed from inside the itemdecl environment, while also showing the name of the item. Idea: @\libglobal{blah}@ and @\libmember{blah}{class}@ (only shows "blah", of course).

[jensmaurer]

@zygoloid, any opinions on the above?

[jensmaurer]

Editorial teleconference: Also want a macro for constructors and similar funny ways. Otherwise, go for it. Apply third bullet only for synopses for the time being.

[jensmaurer]

See also #2223 with the same thrust.