Framing module : adding a feather option ?

[DaveInDev]

Is your feature request related to a problem? Please describe.
The framing module is very useful, but it would be very nice to have a feather option that would allow gradient borders like the one shown on the picture below.

Describe the solution you'd like
A "feather %" slider could be added under the "border size %" slider.
By default it would be 0.
But if non nul, it would add an inner gradient on the border of the picture, using the "border color".
The gradient would range from [border color / alpha 1.0] to [border color / alpha 0.0] = transparent.

Note that the classic opaque border added around the picture will remain identical. The gradient border will be added inside this existing opaque border.