Simplify the build system by changing the repository layout to
follow the recommended Qt module structure.

The Enginio library is now a proper Qt5 module so it can take
advantage of the convenience provided by the module system.
Fix linking of the QML plugin on mac.

Also change the module name to enginio and replace the io/engin plugin path
with Enginio.
Merge build system changes from dev branch
Force ordered build to ensure that the library is built before the plugin to fix build issues with multiple jobs.