[stable-1.2] Support bundling with Qt4

@tresf This is a practical issue, our Macports fixes for bundling script were done against an archaic version, and substantial changes occurred since then in the sources.
Currently bundling script assumes Qt5: https://github.com/LMMS/lmms/blob/stable-1.2/cmake/apple/install_apple.sh.in
Since `stable-1.2` supports Qt4 otherwise, I hope this can be fixed :)

In particular, I am not sure what to do about `Carla`: it is supposed to be a standalone (?) app, but there is no code there to create an app bundle for it.

I also do not get why this is done: https://github.com/LMMS/lmms/blob/d87917cdc9d4777784a643163cbd5f264079c9b6/cmake/apple/install_apple.sh.in#L31
As is, it tries to write directly to prefix, which obviously fails due to permissions, but even if changed to destroot, it makes no sense to dump a random plist into the root directory. Perhaps it should go instead into the app bundle, together with other stuff?

P. S. In the worst case we can implement bundling from scratch on our end, provided I have a ready package (so that I can see what goes where), but that will leave the problem unsolved for anyone trying to use 1.2 without Macports, but with Qt4.

Start with cmake/apple/install_apple.sh.in on stable-1.2 and compare its Qt5 assumptions with the branch's Qt4 support. Investigate how Carla is expected to be bundled and review the referenced plist handling near line 31. Done means Qt4 bundling works for the supported applications without writing an unrelated plist to the prefix root.

Written by the indexing model from the issue text.