UI: new icon set by Andrzej Ambroz.

This is a monochrome icon set, with a more modern look and icons for
various features that did not have a proper icon before.

release/scripts/startup/bl_ui/properties_mask_common.py

@@ -94,8 +94,8 @@ class MASK_PT_layers:
 
         sub = row.column(align=True)
 
-        sub.operator("mask.layer_new", icon='ZOOMIN', text="")
-        sub.operator("mask.layer_remove", icon='ZOOMOUT', text="")
+        sub.operator("mask.layer_new", icon='ADD', text="")
+        sub.operator("mask.layer_remove", icon='REMOVE', text="")
 
         if active_layer:
             sub.separator()