Add a new preference to force fixed size on all notes to prevent
resizing. Also fix a bug where notes taller than the screen height would
not display scrollbars after applet restart.

Fixes #370

When a network interface is disabled at the kernel level, the applet
tries to read from a device that no longer exist, generating continuous
warning messages in syslog.

Check if the device exists before calling glibtop_get_netload in both
is_dummy_device and get_device_info. When the device doesn't exist,
get_device_info returns a non-running device info, allowing the existing
auto-change logic to find an active device.

Fixes #688