adapt "Save as Classification" also for vector layers

[Andreas Janz]

@Sebastian van der Linden @Benjamin Jakimow @Fabian Thiel shall we also allow to create a raster classifications from a styled vector layer?

Like in the raster case, the user would style the layer, which can then be saved as a raster classification with proper colors and names. The only difference would be, that the user has to select a raster that defines the pixel grid for rasterizing the geometries.

This can then also be connected to the DataViews context menu.

If nobody says NO, I will implement this as well .

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[Benjamin Jakimow]

I don’t know how frequent user might use this option, but feel free to add it to a specific “Save as Raster Classification” layer-tree context menu, as described #428.

[Benjamin Jakimow]

Definitely we should. That’s basically the “allow any vector attribute to become a raster classification value” solution I always liked to have.