The Cell::symbol field is now accessible via a getter method (`symbol()`). This will
allow us to make future changes to the Cell internals such as replacing `String` with
`compact_str`.
Fixes issue with inserting content with height>viewport_area.height and adds
the ability to insert content of height>terminal_height
- Adds TestBackend::append_lines() and TestBackend::clear_region() methods to
support testing the changes
