startup-notifications: keep sequence around for 30s after completion

startup-notifications: keep sequence around for 30s after completion

This changes the fact that Firefox would not be launched on the correct
workspace because it marked the startup sequence as completed *before*
actually mapping all of its windows.

To test this, go to workspace 3 and run this command in a terminal:
    i3-msg 'exec iceweasel; workspace 4'
That will make i3 start iceweasel (and create a proper startup
notification context for it), then immediately switch to workspace 4
(before iceweasel could possibly start).

The iceweasel window(s) should appear on workspace 3.