app: add GimpLayerModeContext enum

A bitmask, specifying in which contexts a layer mode is applicable.
Can be a combination of:

  - LAYER: usable as a layer mode for actual layers.
  - GROUP: usable as a layer mode for layer groups.  Currently, all
    modes that specify LAYER also specify GROUP, and vice versa,
    but the planned pass-through mode will be GROUP only.
  - PAINT: can be used as a paint mode.
  - FADE: can be used for fading.

Add a 'context' field to _GimpLayerModeInfo, and provide context
masks to all the modes.

Use the context mask for validation when setting a layer's mode.
The next commit will use the mask when populating the layer mode
menus.

app/operations/operations-enums.h

@@ -157,6 +157,21 @@ typedef enum  /*< pdb-skip >*/
 } GimpLayerModeGroup;
 
 
+#define GIMP_TYPE_LAYER_MODE_CONTEXT (gimp_layer_mode_context_get_type ())
+
+GType gimp_layer_mode_context_get_type (void) G_GNUC_CONST;
+
+typedef enum  /*< pdb-skip >*/
+{
+  GIMP_LAYER_MODE_CONTEXT_LAYER = 1 << 0,
+  GIMP_LAYER_MODE_CONTEXT_GROUP = 1 << 1,
+  GIMP_LAYER_MODE_CONTEXT_PAINT = 1 << 2,
+  GIMP_LAYER_MODE_CONTEXT_FADE  = 1 << 3,
+
+  GIMP_LAYER_MODE_CONTEXT_ALL = (GIMP_LAYER_MODE_CONTEXT_LAYER | GIMP_LAYER_MODE_CONTEXT_GROUP | GIMP_LAYER_MODE_CONTEXT_PAINT | GIMP_LAYER_MODE_CONTEXT_FADE)
+} GimpLayerModeContext;
+
+
 /*
  * non-registered enums; register them if needed
  */