Bump the version to 3.0.0

At the same time, change the library sonames for -3.0 to just -3.
This is necessary since the 2.99 releases installed libraries like
libgtk-3.0.so.0.9903.0, and we want to prevent the library version
number from jumping back. So 3.0 will have libgtk-3.so.0.0.0.