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.

selection.c

@@ -224,7 +224,7 @@ selection_to_text(const struct terminal *term)
         &extract_one_const_wrapper, ctx);
 
     char *text;
-    return extract_finish(ctx, &text, NULL) ? text : NULL;
+    return extract_finish(ctx, true, &text, NULL) ? text : NULL;
 }
 
 void