Replace the 'with-behind' and 'with-replace' properties with a
single 'context' property, and use it to select the included
layer modes, according to their context mask.

Add a dummy GIMP_LAYER_MODE_SEPARATOR value to the GimpLayerMode
enum, and use it to explicitly mark the menu separators in the
layer-mode group arrays; add separators to the layer-mode menu
accordingly.

Update the rest of the code to use 'context' instead of 'with-behind'
and 'with-replace'.  In particular, in the layers and layer options
dialogs, select the right context based on whether or not the
selected layer is a group.