extract: add extract_finish_wide(), and optionally skip stripping trailing empty cells

extract_finish() returns the extracted text in UTF-8, while
extract_finish_wide() returns the extracted text in Unicode.

This patch also adds a new argument to extract_finish{,_wide},
that when set to true, skips stripping trailing empty cells.

terminal.c

@@ -2917,7 +2917,7 @@ rows_to_text(const struct terminal *term, int start, int end,
     }
 
 out:
-    return extract_finish(ctx, text, len);
+    return extract_finish(ctx, true, text, len);
 }
 
 bool