custom processing parameter widget for selecting a band list with bands from multiple rasters

[Andreas Janz]

The QgsRasterBandComboBox allows the selection of a band list with bands from a single raster. Implement a similar widget, that allows to select bands from multiple rasters.

Use this widget for the VRT band math algorithm.

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[Andreas Janz]

Also, such a widget would be great for a band stacking algorithm. Note that GDAL BuildVRT is not fully capable to build a band stack with bands from different sources, because it can only use the i-th band from each source, which is very limiting.