* Added modest-scroll-text.[ch]

* Added modest-scroll-text.[ch]
        * It's a refactorization from ModestRecptView. Now the feature
          "limit the recpt view to two lines" and "scroll line by line"
          is in this class.
        * It inherits from GtkScrolledWindow, and embeds a GtkTextView
        * Generalized to be able to limit the text view to an arbitrary
          number of lines (not only 2).
* modest-recpt-view.[ch]
        * Updated to inherit/use the new ModestScrollText class.
* modest-mail-header.[ch]
	* Now all headers use ModestScrollText (and then they're limited
	  all to two lines, in special subject).

pmo-trunk-r899