Password dialog always has transient parent: Bug #720769

Not only a transient parent, but the main window is shown and
presented if hidden to ensure everything is displayed.  This ensures
the password prompt is associated with Geary, answering some of the
concerns in bug #739195.
Only show one password prompt at a time: Bug #714597

A nonblocking mutex is used to ensure that only password dialog
is displayed at a time.  The prior commit also made the dialog modal.
Identify password prompt as Geary: Bug #739195

Although commit for bug #720769 ensures that the password dialog is
transient for Geary's main window, this text change ensures the user
knows the password being asked for is for Geary.
Password dialog improvements

In addition to changes in prior three commits, this updates the
password prompt UI to closer match the new GNOME HIG.  It uses
dim-label labels and suggested-action for the OK button, whose text
is changed to "Authenticate".  It also cleans up the inter-widget
spacing, which was a bit excessive.