Add Font Ligature Support

This work would not have been done without @Nucearo 's efforts:
        #470

Uses QTextLayout to render segments of similarly styled cells. QTextLayout
provides greater control over Qt's font-rendering engine via QGlyphRun. With
QGlyphRun, we are able to identify individual characters and their positions.

QGlyphRun should provide a means of correcting the spacing for non-perfect
monospace fonts. It also allows for identification of when a character
sequence is converted to a glyph.

src/gui/shellwidget/cell.cpp

@@ -23,6 +23,10 @@ bool Cell::IsStyleEquivalent(const Cell& other) const
 		return false;
 	}
 
+	if (IsDoubleWidth() != other.IsDoubleWidth()) {
+		return false;
+	}
+
 	return m_highlight == other.m_highlight;
 }