2.4 compatibility (Version 3.4)

As you already know, I had a hard time updating it with the current 2.4 collapsible and duplicator plugin.

This PR, https://github.com/symphonycms/symphony-2/pull/2121, includes some css fix and also compatibility with the new state of the plugins with the aforemention PR.

What I did is include #115 in it and create version 3.4, compatible with Symphony 2.5. Is this ok ?

Again, lots of commits, but they are really small and well documented, IMHO.
I'm not sure if it's required by sematic versioning but it makes the scheme more clear.