Remove old forward declaration.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85258 b8fc166d-592f-0410-95f2-cb63ce0dd405
Improve some id_manager comments.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85259 b8fc166d-592f-0410-95f2-cb63ce0dd405
Clean header id generation.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85260 b8fc166d-592f-0410-95f2-cb63ce0dd405
Clean final id generation.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85261 b8fc166d-592f-0410-95f2-cb63ce0dd405
Extract 'order' from `id_placeholder`.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85262 b8fc166d-592f-0410-95f2-cb63ce0dd405
Extract `id_data` from `id_placeholder`.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85263 b8fc166d-592f-0410-95f2-cb63ce0dd405
Combine 'child' and 'unresolved' id states.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85264 b8fc166d-592f-0410-95f2-cb63ce0dd405
Pull the resolved state from `id_placeholder`.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85265 b8fc166d-592f-0410-95f2-cb63ce0dd405
Stop storing generated ids in `id_placeholder`.

Placeholders are now completely constant.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85266 b8fc166d-592f-0410-95f2-cb63ce0dd405
Improve the handling of file ids.

Instead of messing around with the section's id, leave it alone and
check for a file id whenever looking up the current id.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85267 b8fc166d-592f-0410-95f2-cb63ce0dd405
Mark up const use of `id_state` and `id_placeholder`.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85268 b8fc166d-592f-0410-95f2-cb63ce0dd405
Make entity data const where possible.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85269 b8fc166d-592f-0410-95f2-cb63ce0dd405
Split up the id manager file.

git-svn-id: https://svn.boost.org/svn/boost/trunk/tools/quickbook@85270 b8fc166d-592f-0410-95f2-cb63ce0dd405