files-view-dnd: Improve filename generated from dropped text

Make several improvements:

1) Max filename was too long at 128 characters, half it to 64
2) Tweak min filename to 8 characters (still meaningful)
3) Stop at the first sentence rather than last sentence. A filename
should be short and concise, multiple sentences don't make sense.
4) Start at the start_sentence (i.e. eliminate leading whitespace)

Importantly this patch eliminates the potential inclusion of newlines
which is problematic in the views (it causes more than 3 lines of the
filename to appear because of the GtkLabel behavior).