Toolbar buttons missing their label attribute

Initially, the #new-identity-button and #security-level-button xul elements are missing their label attribute. For #new-identity-button we should use the same text as the tooltip. For #security-level-button, probably just "Security Level" is an appropriate label.

Note for #new-identity-button the label is set when the user opens customization, so will be set after this point.

This is not a major usability issue because most of the time the icon and tooltip are sufficient. And for non-visual users, the tooltip text will be used as a fallback accessible name. However:

• It breaks the convention with the other toolbar elements.
• It becomes visually noticeable when the buttons end up in the overflow menu (by shrinking the window width), and only the icon is shown.
• The #security-level-button it has no name during customization when it is not in the toolbar.