Fix include file regex

Fixes #985

Of course the backslash has to be escaped doubly.
Before this commit the regex actually read a "\i" escape sequence,
which is unknown and raises an error starting with Python 3.6

frescobaldi_app/open_file_at_cursor.py

@@ -33,7 +33,7 @@
 import browseriface
 
 # regular expression for finding \include expressions
-incl_regex = re.compile('(\\include\s*\")(.*)(\")')
+incl_regex = re.compile('(\\\\include\s*\")(.*)(\")')
 
 def includeTarget(cursor):
     """Given a cursor determine an absolute path to an include file present below the cursor.