Resolving an inconsistency between toolbar actions and
the configuration window: the view toolbar provides an
action to toggle all deep-sky objects on/off, but in the
configuration window, there were only controls for
individual catalogs (Messier, NGC, IC, etc).

I added a Checkbox to the configuration window to toggle
all deep-sky objects. Unchecking this box will disable
all deep-sky related widgets in the window, like the
"Hipparcos star catalog" checkbox does for stars. The
checkbox and the toolbar button are properly synced with
each other.

No new string was added, but there will be a fuzzy: I
adopted the "Deep-Sky Catalogs" string which was used as
a group-box title. The group box now has no title. For
consistency, I also removed the (redundant anyway) "Star
catalog" title from the stars group box. So that string
has disappeared.