Notify user when filesystem turn read-only

When there are I/O errors on disk, the filesystem for '/' will be automatically mounted read-only. Detect and notify user of this scenario.

In the storage module:

• Write a regularly scheduled task with the glib.py module. For a sample see the scheduled task for checking for low memory.
• Write a method to detect if '/' filesystem is mounted read-only. Perhaps use/adapt the existing methods for reading mounted file systems in storage app.
• Create a Notification that user can see in the UI. For a sample see the scheduled task for checking for low memory.