Wings3d dialogs improvement.

These changes/additions have the intention to make easy to add new controls to be used with wings_ask without need to make a pull request for main devs.
This allows plugin writers (like me) to create they own controls to be used on Wings3d dialogs.

It was provided the sample in the wpc_test_ask.erl (in minimal_dialog and larg_dialog). Any wrong control name will result in the same kind of exception as the original code.

Please consider approve these changes.

p.s. sorry by the truncated text in the commit description.