The current shorcut key for "Use multi-line matching" conflicts with
"In Selection" when "Use regular expressions" is enabled.  It should be
convenient if we change it.

We choose 'g' since other letters are already in use:

   u: "Use regular expressions"
   s: "Search for"
   e: "Use escape sequences"
   m: "Mark"
   l: "In Selection"
   t: "Match from start of word"
   i: "In Document"
   n: "Replace & Find"
   a: "Case sensitive"
   c: "Close"
   h: "Replace with"