Fix memory leak for battery tooltips

Instead of returning a const string, that is fed into strdup, tooltip
functions are now supposed to return allocated strings. This fixes a
memory leak in the battery tooltip.

This is used instead of simply freeing the memory in the battery
tooltip function, since it also avoids a uselesss strdup().

src/tooltip/tooltip.c

@@ -304,7 +304,7 @@ void tooltip_copy_text(Area* area)
 {
 	free(g_tooltip.tooltip_text);
 	if (area && area->_get_tooltip_text)
-		g_tooltip.tooltip_text = strdup(area->_get_tooltip_text(area));
+		g_tooltip.tooltip_text = area->_get_tooltip_text(area);
 	else
 		g_tooltip.tooltip_text = NULL;
 	g_tooltip.area = area;