loginDialog: only emit session-activated on user action
This is an old patch that I needed to resurrect for downstream here:
https://bugzilla.redhat.com/show_bug.cgi?id=1527145
We didn't origionally push it because Jasper had a different technique that was ostensibly better. His approach didn't end up working though and then the bug languished.
So let's just merge this one now.
Right now we emit session-activated any time the bullet moves in the session menu. That includes at start up 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 (it's not only pointless, but it can least to oscillations)
This commit changes the code to only emit session-activated when the user explicitly activates a session item from the gear menu.