replace title attribute with tooltip

[woylie]

Some components were implemented before the tooltip was added. Instead of real tooltips, they use the title attribute, which can be replaced now.

• replace aria-label attribute in icon and icon_sprite components with visually hidden text
• replace title attribute in time component with tooltip
• replace title attribute in date component with tooltip
• replace title attribute in datetime component with tooltip
• replace link with button in app_bar
• remove title attribute from app_bar
• remove title attribute from action_bar
• guidance or updates for all button components about primary tooltips (i.e. icon-only buttons)
• guidance or updates for all button components about auxiliary tooltips