background syntax checking

This is certainly doable. The syntax completer signals the checker to run if it suspects a problem. Errors are not something that is always underlineable.

I have been contemplating using the gtksourceview left gutter to for difference kinds of bookmarks: user added, search results, syntax errors. This allows me to have have a indication of error or style, and a tooltip to state what the issue is.

The implementation for bug #934653 provides an on demand background check. I do not trust it yet as I see that saving a document tries to queue more than one checker. When threaded checking is released. This solving this issue would entail:
    * Queuing an an instance of CheckerWorker to run every 30 seconds:
      GObject.timeout_add(30000, self.checker_worker.start)
    * CheckerWorker.start return False to run only once. start() will must be smarter to return True to keep going.
    * The save event must be disabled or have no effect when background checking is on.
    * background checking will be enabled from the Tools menu and the menu would remember the state.
    * Checking from the menu must reschedule to background process or terminate it to start an new one
     running from the new moment.