Fix an issue in the MPD module where text failed to render
when raw tags containing markup characters were truncated
mid-sequence after sanitization,
resulting in fragmented XML entities (such as `&` cut into `&am`).
Resolve this by shifting the order of operations to
truncate the raw strings *before* running `sanitize_string`.
To keep `setLabel` decoupled and clean,
we introduce helper methods (`getArtistStr`, `getAlbumArtistStr`,
`getAlbumStr`, `getTitleStr`) inside the `MPD` class,
matching the architectural style of MPRIS.
Additionally:
- Introduce support for the customizable `ellipsis` config option
(defaulting to `…`).
- Switch the truncation logic of the MPD module to be
visual-width-aware using the newly extracted
`waybar::util::utf8_truncate`.
- Decouple the tooltip text from the main label's truncated text.
The tooltip now displays un-truncated, complete metadata
(properly escaped), providing a significantly better user
experience.