Support inline upload of single / multiple files

[Aldus83]

We need to create a way to do inline replacement of single files (i.e. node thumbnail, user avatar...).
This should (probably?) be done using Shadow, with its single display output.

Shadow is very powerful but (still) poorly documented so far; keep in mind that you can select the tabs to appear in shadow by adding javascript parameters. See in example the standard node edit buttons:

The qtag qtag_EDIT
https://github.com/quantacms/quanta/blob/master/engine/modules/core/node/node.qtags.inc#L283

and the JS
https://github.com/quantacms/quanta/blob/master/engine/modules/core/node/js/node.js