Layers/Previous
Switch to the previous layer, ie. the layer below the current one.
Layers/Next
Switch to the next layer, ie. the layer above the current one.
Layers/Move up
Moves up the current layer one level. Ie. it swaps the current layer with the next layer (the layer above the current one).
Layers/Move down
Moves down the current layer one level. Ie. it swaps the current layer with the previous layer (the layer below the current one).
Layers/Toggle visibility
Turns the current layer's visibility on or off.
Layers/Insert
Inserts a new layer right above the current one and selects it as current layer.
Layers/Delete
Deletes the current layer. If the current layer is the only one layer, it doesn't do anything.
Layers/Merge down
Merges the current layer with the first visible layer below. The current layer must be visible as well. The layer below will be updated and the current layer deleted. The updating always treats the first color as the transparent color.