Bump version numbers to 1.2.0

As the scope of 1.2.0 has been finalized, and the release branches have been created, the version numbers of all modified components need to be bumped to 1.2.0.

I don't believe any changes are planned for libguac in this release, but in case it's relevant:

In the case of libguac, the SO version (libtool version info) must be incremented only according to strict guidelines). It is independent of the project version.