add "Export as ENVI" button to data sources context menu

[Andreas Janz]

• uses Translate raster layer algo
• takes special care of ENVI header file (wavelength, fwhm, class colors, file type, etc.)

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[Andreas Janz]

should replace old algo:

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[Andreas Janz]

Instead of writing an existing raster in ENVI format, you can now just append a proper ENVI header *.hdr sidecar file next to an existing GeoTiff. Use the Append ENVI header to GeoTiff raster layer algo or the Append ENVI header context menu option in the data sources panel.

Only if the raster isn’t a GeoTiff (e.g. VRT), you need to translate the raster explicitely into a GeoTiff first and append the ENVI header afterwards.

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[Andreas Janz]

resolves #742

→ <<cset 626a21b2de3c>>