Improvements to the chat links system

[Ellypse]

The new chat links system could be improved in several ways, to be more robust, offer better user feedback and use fewer resources:

• Generate tooltip content on link creation and save it in a table once, instead of calling function to generate tooltip text for each request
• Save raw content to send once instead of fetching for each request
• Compress data sent
• Compute the size of the data when sending the first link info and have the size of the thing being send indicated in the tooltip
• Add a loading indicator to the tooltip when it is first fetching the tooltip data
• Add a loading bar (or indicator if a bar is not possible or too hard to do) to the tooltip when using an action button like opening or importing something.

[Ellypse]

New things to do:

• Add support for sending other people's links when shift-clicking a link in the chat frame to insert it in the edit box. We should support re-routing requests for link content to original author.
• Timeout for fetching the tooltip content and when transfering data (if the sender is disconnected or /reload during a download, the download is stuck)