Drag-and-drop sound effect on drop

## Problem

Right now the only confermation of the Drag and drop is the item (hopefully) showing up wherever the user dropped it, which is dumb because sometimes the item may be added to a list of similar things, like in a file browser or a text file. The user needs more tha just sometimes getting visual feedback.

## Proposal

The like in the elementary HIG, there should always be confirmation on the smallest of things, and since there is no way of implementing toasts in every aspect of the OS, especially in third-party apps, some kind of audio feedback for DnD would fill the gaps. I'm not sure if this is possible or not, but since Gala seems to have a lot of control over DnD, trying to detect when an item was dropped over an interactive area could be a good start. As for the sound itself, a small 'plop' may work(? macOS's is a bit overkill)

## Prior Art

macOS High Sierra:

https://user-images.githubusercontent.com/58219504/104899536-e058ec80-5972-11eb-8580-53b12c96286c.MOV