gui: Only unselect range from first data sheet when data changes.

commit 5080068 "gui: Unselect data in data editor when the datasheet
is replaced" unselected all the data in each of the possible four
datasheets (supposing that the "split" view is selected), but if the
"split" view is not in effect then three of the data sheets don't have
windows and this results in "Gdk-CRITICAL **:
gdk_window_invalidate_rect_full: assertion `GDK_IS_WINDOW (window)'
failed".

This commit switches to unselecting the range only from the first data
sheet, which seems to be just as effective at solving the original
problem and does not cause Gdk-CRITICAL messages.