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 am really glad you caught this one. It was already bugging me. To properly solve it, it would require to add a 'setsize.collapsible' event in the collapsible plugin.

We really need to have separate calls for updating the cache sizes vs actual cssom values.

Would it be ok? I did not originally did it because it would be another public api to support.