Extended the Python plugin to support custom tileset exporters and readers written in Python.

* Un-duplicated setPythonClass function

* Removed unused pluginmanager.h include

* Fixed cleanup code related to reloading plugins in case a plugin
  changes from defining a map format to a tileset format or vice versa.

* Allow a Python plugin to include both a map and a tileset format
  (though only one of each).

It appears Qt Creator's Qbs support does not include looking for compiler
flags setting include paths (-I). They need to be actually set using
cpp.includePaths.

There are actually no defines coming from python3-embed for me.

The problem was that handleError was not called when PyObject_IsSubclass
returned -1, which it commonly did when called with a value that wasn't
a class. With the error indicator still set, the next call to
PyObject_Dir was failing.

Since the errors aren't usually very useful, we just call PyErr_Clear when
PyObject_IsSubclass fails.

I couldn't find a convenient way to check if the value is a class, which
could avoid raising these errors in the first place.

The PyErr_Print call already clears the error indicator, according to
the docs.