Add subclassing support

[mappu]

Fixes: #42

Fixes: #69

Fixes: #80

Subclass all Qt classes containing virtual methods, and add On() wrapper functions to implement the virtual override. This works for all virtual methods including paintEvent and overloaded operators.

C++ virtual subclasses have a different pointer value. Change CABI constructors to return every virtual subclass pointer so that Go types can be accurate.

Add support for calling super() in overloaded callbacks.

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Remaining work:

• Fix remaining compile issues
  • Fix defer free happening too early in miqt_exec_callback_QAccessibleWidget_Relations
• Add super() helper to Go callback
• Check if compile time is reasonable or if other mitigation is required (e.g. move every class into an optional Go subpackage)
  • Seems about 20% slower
• Add example
• Usage guide in the README