fix crash when print from qgis processing algorithm while the python console is displayed

[troopa81]

Description

If user use the method print inside a QgsProcessingAlgorithm and the python console is open, the application crashes. It's because when the console is opened, the print is caught and reported in the console through QScintilla, but GUI stuff has to be done is the main thread.

This PR use the Qt event loop to handle the print in the main thread.

Checklist

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[troopa81]

@nyalldawson does it need to be backported?

[m-kuhn]

@nyalldawson does it need to be backported?

yes please