* .gitignore: 2.6 -> *.
* NEWS: Note that the next release will break ABI.
* README: 2.6 -> 3.0
* configure.ac: Version 2.90.0, 2.6 -> 3.0, reset LIBXMLXX_SO_VERSION to 0:0:0.
* docs/manual/libxml++_without_code.xml: 2.6 -> 3.0
* libxml++/libxml++.h: 2.6 -> 3.0
* libxml++-2.6.pc.in: Rename to libxml++.pc.in.
* libxml++.pc.in: Rename from libxml++-2.6.pc.in. Bug #754673.

The libxml++-2-40 branch is intended for future 2.40.x releases with the
libxml++-2.6 ABI.