Allow enabing pango markup in the taskbar string

The fix for taskbar tooltips in 6a2d214b55 was incomplete: it causes the label
to contain escaped titles.  Use set_markup so that GTK decodes markup again,
but only if requested by the user (disabling markup is needed if using format
strings like "{title:.15}" to avoid terminating the string in the middle of an
XML entity).

man/waybar-wlr-taskbar.5.scd

@@ -32,6 +32,11 @@ Addressed by *wlr/taskbar*
 	default: 16 ++
 	The size of the icon.
 
+*markup*: ++
+	typeof: bool ++
+	default: false ++
+	If set to true, pango markup will be accepted in format and tooltip-format.
+
 *tooltip*: ++
 	typeof: bool ++
 	default: true ++