Workflows: re-export grayscale icons to make guidelines_enforcer's ic…

…on check happy

The glyph build task seems to be working perfectly fine with icons of bit depth
lower than 8 bit-per-pixel, but the guidelines_enforcer enforces 8 bit-per-pixel.

Note that the final app binary size is unaffected by this change, as the glyph
build task reduces the color palette / bits-per-pixel again to the supported
maximum number of shades of gray.

As app_nimiq_64px.gif seems to have slightly changed, this commit also updates
the Ragger screenshots accordingly. At the same time, some unused excess images
have been removed which were not properly deleted from previous tests.

Makefile

@@ -67,7 +67,17 @@ DISABLE_STANDARD_APP_FILES = 1
 #   control, also Colors > Dither might be used.
 # - Set the Layer > Layer Boundary Size to the intended size of the final image, centering the content. Then Image > Fit
 #   Canvas to Layers.
-# - Export the image as gif, which should be saved with the desired bits per pixel, if following these steps.
+# - Save the icon as xcf with the indexed color map which can be used for future changes to the image.
+# - For binary black/white icons: export the icon with indexed color mode as gif, which will save as 1bit-per-pixel.
+#   For grayscale icons: The guidelines_enforcer.yml workflow expects grayscale images to have 8bit-per-pixel depth
+#   (even though the glyph build task seems to be compatible with lower bit-per-pixel), therefore change the Image >
+#   Mode to Grayscale and then export as gif. Do not overwrite the xcf file, to keep it in indexed mode.
+#   For some reason, for glyphs/app_nimiq_64px.gif, but not for the other icons, this made the glyph build task complain
+#   that the image is not indexed. So not quite sure yet, what the proper procedure would be here. Also tried, re-export
+#   of app_nimiq_64px.gif via https://www.photopea.com/, which exports with a color map with 256 entries, excess entries
+#   simply being filled up as black, but that still had 4bpp depth the enforcer complained about. Switching to Grayscale
+#   and then back to an indexed palette with 32 shades of gray seems to have done the trick.
+#   Note that for checking for the bit depth, ImageMagick's identify can be used: identify -verbose /path/to/icon.gif
 ICON_NANOS = icons/app_nimiq_16px.gif
 ICON_NANOX = icons/app_nimiq_14px.gif
 ICON_NANOSP = icons/app_nimiq_14px.gif