Enable styling of children

[Awe42]

Right now, the uploader only accepts a default_style which is used as style for the parent uploader div. It would be really useful if we can pass additional dicts that override the style of children components, like the text label or progress div.

Alternatively, if instead of the "Drag and Drop" text, we can define the children of the label tag through dash components - that would allow a lot of extra options, like adding pictures or splitting long text into multiple lines.