Add javascript event when updating page attachments

Currently the events fired by the attachments manipulation in the footer of a wiki page are incomplete and not necessarily oriented towards allowing other scripts on the page to be notified when the page (attachments) changes, as follows:

• there seems to be no event at all fired when an attachment is deleted;
• for the upload of a new attachment, there are some events that are fired by the html5 uploader, such as xwiki:html5upload:fileFinished - when a file is done uploading - and xwiki:html5upload:done when the upload is done (with page change or cancel).

The javascript APIs for this should be more clear and clearly documented.

Also, they may need to be aligned with other javascript APIs for page change (such as the ones used by inplace editing and property inplace editing).