[libcontentaction] Don't duplicate lca-tool and data in both Qt 4 and Qt 5 versions.

We only need one copy of these, so only build lca-tool for Qt 5 and install both
there. For Qt 4, add a dependency on the Qt 5 version of libcontentaction so
that lca-tool (and stuff requiring the xml data) will continue to work.
Merge pull request #3 from rburchell/master

[libcontentaction] Don't duplicate lca-tool and data in both Qt 4 and Qt 5