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.