Allow polling for changes in watched folder

Some underlying file systems might not trigger change events (e.g.
mounted remote file systems). Added a feature flag to allow for
switching to a (less performant) polling method.
(cherry picked from commit f2df174)

src/octoprint/server/__init__.py

@@ -14,6 +14,7 @@ from flask.ext.babel import Babel, gettext, ngettext
 from flask.ext.assets import Environment, Bundle
 from babel import Locale
 from watchdog.observers import Observer
+from watchdog.observers.polling import PollingObserver
 from collections import defaultdict
 
 import os
@@ -399,7 +400,12 @@ class Server():
 				printer.connect(port=port, baudrate=baudrate, profile=printer_profile["id"] if "id" in printer_profile else "_default")
 
 		# start up watchdogs
-		observer = Observer()
+		if s.getBoolean(["feature", "pollWatched"]):
+			# use less performant polling observer if explicitely configured
+			observer = PollingObserver()
+		else:
+			# use os default
+			observer = Observer()
 		observer.schedule(util.watchdog.GcodeWatchdogHandler(fileManager, printer), s.getBaseFolder("watched"))
 		observer.start()
 

src/octoprint/settings.py

@@ -142,7 +142,8 @@ default_settings = {
 		"repetierTargetTemp": False,
 		"externalHeatupDetection": True,
 		"supportWait": True,
-		"keyboardControl": True
+		"keyboardControl": True,
+		"pollWatched": False
 	},
 	"folder": {
 		"uploads": None,