Brush Library

Other->Brush Library
or click on the icon at the top left near brush bays

Bay Capture / [ bay-c ]
Edit->Brush preset from bay
captures active brush (bay in top panel) into a brush preset

Layer Capture / [ layer-c ]
Edit->Brush preset from layer
creates a brush preset using content of a selected layer. You may use selection tools to isolate area which shall be the tip of the new brush. Otherwise, the whole layer will be sampled.

note: empty / transparent pixels will be truncated. If you would like to preserve empty space - manually create a new preset and choose an image from desktop (in brush tip).

Import allows to add a preset from a file.
Export allows to export selected preset to a file.

Blend Assist option sets default state of brush to blend if checked. Hold Z key to toggle blend assist on the canvas.