Separate default values from initial values in dialogs

[Martin Kou]

Instead of treating the initial values in dialog opening as another set of default values, initial values in a dialog should be stored and deleted as a kind of its own. This removes the need for a default value stack in the dialogui and dialog plugin, also removes the need to "pop" default values when a dialog closes.

[Frederico Caldeira Knabben]

The code looks good, but can you please provide a patch for trunk?

[Martin Kou]

Fixed with [3044].