Remove the icon from the excel export button if a custom label was used

Today, since XALTXLS-17 , it is possible to specify a custom label for the export button.
However, an icon is added to this button for the excel export, regardless of the label configured by the user.

For consistency reasons, and since it's possible to style the button custom to add the icon using a custom CSS, there should not be this icon for the excel export, since there is not one for the csv export.
However, in order to not break compatibilty, we should keep the icon when no specific customization is done by the caller. However, if the caller is configuring the button label explicitly, it can configure the icon as well and no icon should be added automatically.