loginDialog: defer loading user list until idle

In some cases we load the user list after going back
to main loop and in other cases we load the user list
right away (depending on if accounts service is ready).

In the case we load the user list right away we cause a
traceback because loading the user list forces a reset,
which then tries to reset actors which aren't instantiated
yet.

This commit ensures the user list is loaded after the constructor
finishes and the event loop runs irregardless of the accountsservice
state.

https://bugzilla.gnome.org/show_bug.cgi?id=721868
loginDialog: move user list loading after actors are constructed

Right now we queue populating the user list in the middle of setting
up the dialog actors. Of course, the actual population happens some time
later after going back to the main loop.

It's more logical to structure the code so the the actors are
instantiated first in one block and then other things after that.

This commit moves the user list population enqueuing operation to the
bottom of the constuctor.

https://bugzilla.gnome.org/show_bug.cgi?id=721868
switcherPopup: Always show the arrows if the popup is scrollable

Currently we only show/hide the left and right arrows when
we reach the initial/end position. This patch changes this
behaviour by showing the arrows whenever is a scroll is possible.

https://bugzilla.gnome.org/show_bug.cgi?id=711467