Improvements of misc.dialog

- Renamed misc.dialogs module to misc.dialog
- Renamed DialogContainer to ParameterManager
- DefaultParameterManager is now used to get an object with basic
datatypes already registered
- Added Datatype.get_default_value()
- ParameterManager.add() now takes an additional argument
`default_value`.
- Added ParameterManager.set_defaults()
Added resource.Resource.get_symbol_name()