telepathyClient: Only use 12-hour format when supported by the locale

It is a bit odd to request AM/PM format when the locale selected
by LC_TIME lacks the concept. We ignore the format setting in that
case elsewhere and assume 24-hour format, let's do the same for
chat timestamps for consistency.

https://bugzilla.gnome.org/show_bug.cgi?id=728271
windowManager: Remove (un)blockAnimations()

They are no longer used, kill them.
loginDialog: Remove unused variable
workspace: Don't allow closing windows with attached modals

Modal dialogs prevent the parent from being closed in "normal mode",
so it makes sense to not allow it in the overview either.

https://bugzilla.gnome.org/show_bug.cgi?id=729886
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