Add blend mode to status layer widget

[Martin Owens]

The layer information in the status bar does not currently specify the kind of blend mode applied to the layer. This drop down should at least say "Layer 1 (multiply)" if that mode is applying to that layer.

This will reduce the instances of people with 'broken' svg files because they fail to realise that the layer has been set to a blend mode by accident. Giving them information directly in the location it should be.

See widget/layer-selector.cpp:564 for the layer status widget text.

The blend mode information is currently unavailable because of the way blend modes are calculated in the filter ui. ui/widget/filter-effect-chooser.cpp this get_blend_mode functionality should be moved to the layer object so long as we have blend modes as a property of layers and not as an add on filter later.