prune network interfaces that no longer exist

[ThorOdinson246]

Fixes #557.

The problem

Vitals never forgets a network interface. Once one disappears from /sys/class/net,
nothing removes it from _history, _networkSpeeds or _networkSpeedOffset.

Two effects:

• The menu keeps listing interfaces that no longer exist.
• Device rx/tx sums every entry in _networkSpeeds, so a dead interface's last speed keeps being counted.

The fix

When _queryNetwork sees an interface disappear, it reports 'destroy' for that sensor
through the normal callback. The same mechanism is used by a disabled sensor to report 'disabled'. extension.js handles it next to the existing 'disabled' check and drops the menu item, the hot item and the stored history.

A few notes on the choices, happy to change any of them:

• Nothing is written to GSettings. hot-sensors is left alone, so a pinned interface comes
  back if it reappears rather than being silently unpinned.
• Removal triggers on the interface being gone from /sys/class/net, not on it being down. So,
  a down interface keeps its directory, so toggling wifi does nothing here. I avoided matching
  on veth*/br-* names since that would hide real docker bridges.