Add to `Context` a command for `Create a AOI Cartographic Mask`

Okay, this is a really silly, but cartographically useful request. I was inspired for this by looking out my window at the Cinca River Valley Bottom draped in fog... It reminded me of something we regularly do (or actually the opposite of something we do), which is to emphasize the area of interest, or watershed of interest, we create a layer which has a layer that is just a rectangle (probably 30% greater than extents of polygon of interest), and we symbolize it as fill color white, no edge/border color, make it transparent (typically 60%) and sometimes we feather the edge. Below we did this for a watershed as the AOI and it really helps make the area you care about pop!

It is nothing more than steeling a common Adobe Illustrator trick that National Geographic uses (in example below from a poster that shows Colorado River basin with that fade edge):

Doing it is a bigger pain in the ass then you might think (much like creating a hillshade for a DEM #171). To do this, you need:

To derive a polygon of shape rectangle that is 30% bigger than the area of interest you are creating this for.
Invoke the erase command to erase out the doughut of your area of interest.
Add layer (typically to top of tree) with the above described symbology applied.

Riverscapes / QRiS

Add to `Context` a command for `Create a AOI Cartographic Mask` #170