Fix an issue where the mpris module's tooltip failed to render
when track metadata contained unescaped XML/HTML markup characters.
This occurred because
the mpris module formatted raw strings into the tooltip template
before passing the final string directly to `set_tooltip_markup()`,
triggering GTK/Pango parsing warnings and rendering failures.
Resolve this by wrapping tooltip metadata fields
in `Glib::Markup::escape_text` before formatting,
ensuring Pango-compliant strings are always delivered to the GTK
tooltip markup.
Move the UTF-8 visual-width measurement and truncation helper functions
from the mpris module to a common utility
(include/util/utf8_string.hpp and src/util/utf8_string.cpp).
This decouples string truncation and width measurement
from the mpris module, allowing other modules (like mpd) to reuse.
The helper function `utf8_truncate` (formerly `truncate` in mpris)
and `utf8_width` is exported under the `waybar::util` namespace,
while the low-level `measure_and_truncate` (formerly `utf8_truncate`)
is encapsulated in an anonymous namespace in src/util/utf8_string.cpp
to avoid unnecessary API exposure.
No functional changes were made to the mpris module's behavior.
Siriusuna