Add hardcoded macros to code generator

[Ameobea]

For example, instead of having to include

        def pre():
            qapp = QApplication([])

        gen.add_preamble(pre)

for every widget that creates a widget GUI, perhaps it would be better to create a hardcoded macro in the code generator that inserts it automatically or via some code generator parameter. It would save code in the widget files and eliminate duplicate lines.

This could be repeated for other commonly used code segments, further increasing the efficiency of the generated code.

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