#160 Refactor folders: Element, Container, and Exception

Part of #160:
- Rename folder `Section` to `Element`
- Rename folder `Exceptions` to `Exception`
-  Flatten the `Element` folder structure (move `PreserveText`, `Cell`, and `Row` up one level)
- Create `Container` folder and move `Section`, `Header`, `Footer`, `Settings` into `Container`
- Create `Container` abstract class, inherited by `Section`, `Header`, `Footer`, `Footnote`, `TextRun`, and `Cell`
- Allow some elements in containers (e.g. link and list in header/footer, image in footnote)
- Deprecate `createSection`, `createHeader`, `createFooter`, `createFootnote`, and `createTextRun` in favor of uniform `add...` methods.
- Create `Element` abstract class