[Stack raster layers] add option for stacking VRT raster files without relying on the source VRTs afterwards

[janzandr]

Discussed with @jakimowb.

This will expose the existing API functionality #217 to the Stack raster layers algorithm.

Also fix some limitations of the existing API functionality:

• [ ] should work with VRTs created by gdal.Translate
• [ ] should work with VRTs created by gdal.Warp
• [ ] should work with VRTs including Pixel Functions
• [ ] should work with normal raster bands (e.g. from a GTiff)
• [ ] source VRTs can be stored in any folder (currently, all VRTs must be stored in the same folder)
• [ ] user should have the option to choose between using absolute or relative filenames

[janzandr]

• [ ] should work with VRTs created by gdal.Warp

@jakimowb, correct me if I'm wrong, but it seem to be impossible to have multiple raster sources in a single VRTWarpedDataset. So we will skip this feature. If a warped VRT band is part of the bands to be stacked, we will add the original VRT.