Xpra: Ticket #1494: drag and drop: support images, text, server-to-client, etc

Follow up from #1493.

we have the source window and target window, so we should be able to send the data to the specific application window that was pointed to (if it supports it... which we won't know about until it is too late?)
we should handle more data types and sanitize them: the "selection-data" structure can give us plain text, UTF8, images, etc.
this same code could be re-used for sanitizing clipboard data (ie: images)
on the server-side, we may be able to register the "world-window" as a drop target so that we can trigger drag-n-drop events from server to client

Examples of targets with common applications:

browser URL:

'STRING', 'UTF8_STRING', 'text/plain', 'text/x-moz-url', 'TEXT', '_NETSCAPE_URL'

browser text data:

'text/x-moz-url', '_NETSCAPE_URL', 'application/x-moz-custom-clipdata', 'text/unicode', 'text/plain;charset=utf-8', 'text/plain', 'text/html'

browser image:

STRING, UTF8_STRING, XdndDirectSave0, text/plain, text/x-moz-url, TEXT, text/html, chromium/x-renderer-taint, application/octet-stream

Docs:

Links:

Related to #812.