Install extra/ directory, and configure .natvis files on Windows

[k3DW]

Follow-up of #81

[pdimov]

Let's leave BOOST_INSTALL_INCLUDE_SUBDIR alone and (always, regardless of layout) install into CMAKE_INSTALL_DATADIR/boost-1.90.0.

I'll take a look at the rest later.

[k3DW]

It makes more sense to have matching installation for include/ and extra/. Using BOOST_INSTALL_INCLUDE_SUBDIR avoids that issue. For example, either we get include/boost-1_90 and share/boost-1_90, or we get simply include and share. I think they should always match.

[pdimov]

No, I don't think it does. The include subdir is maintained for consistency with b2 install, new things (e.g. the CMake config files) use the -1.90.0 convention.

I don't think that dumping all the libs/libname/extra directories into /usr/share is what we want (or what the user would have wanted.) We can afford to do this for libs/libname/include only because their contents always have a boost subdirectory, usually boost/libname subdirectory, and collisions are checked by b2 headers. We don't have any established conventions for the contents of extra, or any enforcement or checks for those nonexistent conventions.

So to avoid dumping random things into /usr/share, we should install in a subdirectory, and to avoid libraries stepping over one another's toes, we probably should install into boost_libname-1.90.0, like the CMake configs.