Modernize Python bindings

The current Python plugin relies on a large amount of generated code based on the somewhat hard to maintain and always incomplete tiledbinding.py and qtbinding.py.

I've recently learned about pybind11 (see also this blog post), which seems like a much nicer way of binding the classes to Python. The bindings are defined with similar verbosity, but directly in C++ which makes it easier to keep up-to-date.

In addition, pybind11 provides a more convenient way of interfacing with an embedded Python interpreter than using the Python API directly.