GlobalActions: Fix possible race condition when showing dialog.

Depending on the presence of a previous dialog, the newly generated one
gets either immediately displayed (no old dialog) or shortly delayed
(after dismissal of the existing one).
If a new show event happens (by rapid fire of the power key, for instance,
using a keyboard with fast-repeating events) during that delay, a new
dialog will be immediately displayed, and when the delayed show triggers
it'll conflict with it (and bring everything down).

This moves the creation of the new Dialog object to _before_ showing
it (delayed or not), instead of the show method itself, to ensure the
object isn't clobbered by competing/sequential calls to handleShow()
Enable multi-user support for volume_link_notification