Improve efficiency of diagnostic marker drawing

[techee]

At the moment, diagnostic markers (error/warning underlines) are redrawn every time a message arrives from the server. Also, all the document's markers are cleared and all of them are re-inserted. This could be made more efficient if there are performance problems:

• marker re-drawings could be batched and performed only after some small delay in case multiple diagnostic messages arrive in burst from the server
• some diff could be calculated against the displayed markers and only the differences could be cleared/redrawn
• possibly, only the visible area of the document could be redrawn