loginDialog: only emit session-activated on user action

loginDialog: only emit session-activated on user action

Right now we emit session-activated any time the bullet
moves in the session menu. That includes at startup when
picking an item arbitrarily, and any time GDM reports the
session was read from the user's account settings.

session-activated informs GDM about the newly selected session,
so emitting it in response to GDM reporting a session is a
bad idea.

This commit changes the code to only emit session-activated when
the user explicitly activates a session item from the gear menu.

Note, we no longer set the active session explicitly at start up.
This is a good thing since the item we were picking wasn't
necessarily correct.  It does means if GDM fails to inform us
about the correct default session we'll now show no bullet instead
of a bullet on the wrong item.

https://bugzilla.gnome.org/show_bug.cgi?id=740142