xgettext - PHP multiline strings

xgettext doesn't consider more texts concatenated using the dot operator in the PHP script as a single string and thus it splits them as separate entries in the .po file.

This patch fixes this bug. There are several limitations:

• Only quoted texts can be concatenated (they can contain variables), it's NOT possible to concatenate a quoted text and a variable
  
  RIGHT: $x = "text1" . "text2 $variable";
  
  WRONG: $x = "text1" . $variable;


• In case of multiline texts, it's NOT possible to use a comment between the parts of the text
  
  RIGHT: $x = _("text1".
  
  "text2");

WRONG: $x = _("text1". // first line

"text2");

• Only texts quoted with the same type of qoutes can ce concatenated.
  
  RIGHT: $x = "text1" . "text2"; // double-quotes
  
  WRONG: $x = "text1" . 'text2'; // double-quote and single quote

Althou there are few limitations this patch is quite good as it allows to split long texts to more lines and concatenate the with the dot.