Support a separate (monochrome) atto branding image.

[abias]

Hi,

thank you for implementing the branding feature in b1ffc62.

Within this feature implementation, atto_edusharing gets the icon from the edusharing/appicon setting of mod_edusharing.
However, you might be aware that module plugin icons are colored but atto plugin / button icons are monochrome normally.

The atto_edusharing button has been sticking out of the atto toolbar up to now anyway as it used a blue icon, but this would be the chance now to change that at least for branded setups of atto_edusharing.

I would be grateful if you could improve the branding feature in a way that atto_edusharing gets its own admin setting with which the admin can upload a monochrome icon for the atto toolbar.

Thank you very much.