Web activities should be aware of the session

[manuq]

and stop cleanly in reboot, shutdown, logout.

TestCase:

• start sugar
• start Gears web activity
• make changes
• logout sugar (without manually closing Gears before)
• restart sugar
• restart the previous instance of the Gears web activity

Expected: last changes in Gears instance should be there.

Discussion started at: ​https://github.com/sugarlabs/sugar-toolkit-gtk3/pull/29

WIP patch: ​https://github.com/manuq/sugar-1/commit/a1e96c8661b103ec90b8afb4658aadc207667e7a

[manuq]

​https://github.com/sugarlabs/sugar/pull/90

[Manuel Quiñones]

SessionManager: make it work with web activities

Fixes #4611 .

SessionManager now sends a shutdown signal in each logout, shutdown or
reboot. Apisocket connects to this signal and sends the stop message
to web activities, so they can do cleanup and store.

SessionManager now uses the ShellModel to check if all activities have
been removed. If not, it tries again after one second for ten times.
After that, it forces shutdown.

SessionManager has been moved completly to the shell. It was
inheriting a counterpart class in the toolkit, but was the only
consumer of that class.

Journal is a special case. It will not be destroyed automatically
like other activities. So we connect it to the SessionManager
shutdown signal too, and destroy it in the callback.

Changeset: 69961112e9dbdb79cbae06deb3d876dde74d33bc