Improve default behaviour of generated UIs

Update `EpicsGUIOptions`
- `output_path: Path = Path("./opis")`
- `name: str = "fastcs"`
- add `full_file_path` property that returns `f"{output_path / name}{file_format}"`

Update `EpicsGUI.create_gui` to use `full_file_path`

## Acceptance Criteria
- Update all usages, tests and demo
- All screens and sub screens should appear in `output_path`

Start with EpicsGUIOptions and EpicsGUI.create_gui, then locate all usages, tests, and the demo mentioned in the issue. Run the relevant tests and verify that generated screens and sub screens appear under the configured output_path.

Written by the indexing model from the issue text.