implement a custom processing widget for selecting pickle files

[Andreas Janz]

When using processing algorithms that require the selection of fitted models, the user has to manually browse to the file location, e.g.

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It is proposed to implement a custom widget that allows to dirctly select pickle files available in the Data Sources panel:

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[Andreas Janz]

Done, can now be used e.g. to select existing models in prediction:

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[Andreas Janz]

resolves #1005

→ <<cset 6e5fe07f729f>>