Feature: Use placeholder icon in legend for raster layers

A new feature has been added to allow the selection of a placeholder image which may be used in the legend item of a raster or vector layer. This is very useful in certain instances such as when using raster layers with QGIS server, preventing long lists of palleted values from GetLegendGraphic requests and instead returning a tailored result, among various other use cases.

This feature was funded by Canton of Glarus

This feature was developed by mhugent