After selecting a layer, click again for a pop-up layer menu panel. 

Copy Layer

Click to duplicate the current layer.

Merge Layers

Click to merge the current layer and the one below.

Merge All

Click to merge all layers.

Clear

Click to clear the current layer.

Lock Transparent Pixels

Click to lock the transparent pixels of the current layer so that you can only paint in pixels that are opaque.

Delete Layer

Click to delete the current layer, which can be retrieved by undoing the operation.

Layer Mask

Click to create a new layer mask for the current layer.

Opacity

Drag the slider to adjust the opacity value of the current layer.

Blending Mode

Select to blend the current layer and the one below, you can choose among 24 blending modes.