Painting a soft selection
You can paint a selection however you want. Just paint with a colour (I often use a bright green) on a separate layer. Then use the Selection tool to produce sharp outlines, or Gaussian Blur to make it soft. When you’re finished, just slide the layer to the left and choose Select. This will make a selection of the layer’s content. Then turn the green layer off and select the layer you want to apply the selection to.