app: derive the layer-mode menu layout entirely from the info arrays
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.
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