The user_data used in the dbus signal callbacks should remain accessible
until the last callback fires and terminates.
While this is usually achieved by adding a ref to the user_data object
(e.g. g_signal_connect_object), the GsdPowerManager object clears its internal
structs before being finalized, in gsd_power_manager_stop.
So instead, we must make sure to disconnect the signal callbacks which might
use the manager's internal structs before freeing such structs.

This should hopefully fix the memory corruption that is happening in
issue #833. At least the current code is plainly unsafe.

Fixes #833

The service may not be installed, or have failed to start, or we may
have failed to connect to the bus.

Rather than crashing later, bail out as soon as we find out.

plugins/power/test.py:842: SyntaxWarning: invalid escape sequence '\('
  self.p_notify_log.check_line_re(b'[0-9.]+ Notify "Power" .* ".*" ".*Wireless mouse .*low.* power.*\([0-9.]+%\).*"', timeout=0.5)