Include double square brackets when dragging titles

Dragging a tiddler link into a tiddler editor, or outside the browser
into another app, will now add double square brackets around the title
if it includes spaces. Suggested by @tgirod.

I’m not 100% sure about this change. It breaks one habit that I had
developed: typing `[[sometext|]]` and then dragging a title in between
the vertical bar and the first closing square bracket. What do others
think?