Setting the default icon name causes that it's applied globally to all
newly opened windows in the process, not just for the applet's windows,
so it cannot be used if the applets are built as in-process.

The translation domain needs to be set before loading GtkBuilder files
to get the translated strings when the applets are built as in-process.

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