Update variables and fix Qt5 executables

Replace deprecated `OUTPUT_PATH` variables with `OUTPUT_DIRECTORY` equivalents as discussed in b74a4ecb4ab94db583c133982d29842d0a1a1cbf and fix Qt5 executables and plugins.

On Windows, DLLs are considered RUNTIME, and therefore from now on they will end in the `bin` directory (nothing changes when compiled STATIC or on other platforms"). This also has the advantage that user needs to add only one directory to the `PATH` environment variable.

On multi config generators, QML plugins are now built in `lib/qt5/qml/<CONFIG>/` and the executables import both from `../../lib/qt5/qml/<CONFIG>/` and  `../lib/qt5/qml/` in order to work both for install and build trees. (EDIT: fixed relative paths)

@mbrunettini This will probably have impact on the packaging scripts, since dlls no longer need to be copied in the bin directory.