Autoreload Nagview (#942)

* Modified autoreload to use new dirwatch infrastructure, and added in nagview to verify that fs changes don't stomp on our changes, unless you want them to.

* Split out reload functionality to actual document, and added in a thread to check the document, in the cases where it wouldn't be covered by dirwatch.

* As per request from jgmdev, added in ability to show nagview always.

* Changed things over to use dirwatch.

* Made sure we redrew things, added in a contingency in 'save' for times when we load a non-existent file, and added some checks.

data/core/dirwatch.lua

@@ -31,10 +31,15 @@ end
 -- In windows, this is a no-op for anything underneath a top-level directory,
 -- but code should be called anyway, so we can ensure that we have a proper
 -- experience across all platforms. Should be an absolute path.
+-- Can also be called on individual files, though this should be used sparingly,
+-- so as not to run into system limits (like in the autoreload plugin).
 function dirwatch:watch(directory, bool)
   if bool == false then return self:unwatch(directory) end
+  local info = system.get_file_info(directory)
+  if not info then return end
   if not self.watched[directory] and not self.scanned[directory] then
     if PLATFORM == "Windows" then
+      if info.type ~= "dir" then return self:scan(directory) end
       if not self.windows_watch_top or directory:find(self.windows_watch_top, 1, true) ~= 1 then
         -- Get the highest level of directory that is common to this directory, and the original.
         local target = directory