BStringView: add support for multiline strings

* Actually draw the string at the bottom of the frame.
* Unfortunately BStringList cannot be cached because there is no
  space left in the class.
* Change SGI and PNG translators to use it in place of BTextView.

Change-Id: I07e12bf1a8dc956d18c9624604c7b63453ad15a2
Reviewed-on: https://review.haiku-os.org/620
Reviewed-by: waddlesplash <[email protected]>

headers/os/interface/StringView.h

@@ -79,6 +79,7 @@ private:
 
 private:
 			BSize				_ValidatePreferredSize();
+			float				_StringWidth(const char* text);
 
 private:
 			char*				fText;