fix(mpd): truncate before sanitization and improve tooltip UX

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.

include/modules/mpd/mpd.hpp

@@ -33,6 +33,7 @@ class MPD : public ALabel {
   detail::unique_status status_;
   mpd_state state_;
   detail::unique_song song_;
+  std::string ellipsis_;
 
  public:
   MPD(const std::string&, const Json::Value&);
@@ -45,6 +46,10 @@ class MPD : public ALabel {
   void setLabel();
   std::string getStateIcon() const;
   std::string getOptionIcon(const std::string& optionName, bool activated) const;
+  std::string getArtistStr(bool truncated) const;
+  std::string getAlbumArtistStr(bool truncated) const;
+  std::string getAlbumStr(bool truncated) const;
+  std::string getTitleStr(bool truncated) const;
 
   // GUI-side methods
   bool handlePlayPause(GdkEventButton* const&);