Formalize zero-width emote implementation (#4314)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/widgets/splits/SplitHeader.cpp

@@ -960,8 +960,7 @@ void SplitHeader::enterEvent(QEvent *event)
         }
 
         auto *tooltip = TooltipWidget::instance();
-        tooltip->clearImage();
-        tooltip->setText(this->tooltipText_);
+        tooltip->setOne({nullptr, this->tooltipText_});
         tooltip->setWordWrap(true);
         tooltip->adjustSize();
         auto pos = this->mapToGlobal(this->rect().bottomLeft()) +