loginDialog: Move long session-list menus to the side

Currently the menu position below the button means that the menu
can extend to roughly half the screen height before ending up partly
off-screen. This is plenty of space for commonly installed sessions,
but some users have a significantly higher number of sessions in the
list. Move the menu to the side of the button in that case to maximize
the vertical space the menu may take up.

https://bugzilla.gnome.org/show_bug.cgi?id=734352