Set ed.keydelay when backspacing an empty line

Similar to 2ebccbc3e9, there's also a
long-standing bug where if you backspace an empty line (and this time,
the line IS actually already empty, not merely
emptied-earlier-in-the-frame), the game will quickly delete more blank
lines if there are any above the blank line you deleted. Again, this is
annoying too, if you so happen to need to use lots of blank lines.

To fix this, it's simple - just set ed.keydelay to 6 when the game
backspaces an empty line. Then it won't be so trigger-happy in deleting
blank lines.

desktop_version/src/editor.cpp

@@ -3870,6 +3870,7 @@ void editorinput( KeyPoll& key, Graphics& dwgfx, Game& game, mapclass& map, enti
                     }
                 }
                 key.keybuffer=ed.sb[ed.pagey+ed.sby];
+                ed.keydelay=6;
             }
 
             ed.sb[ed.pagey+ed.sby]=key.keybuffer;