Redo of D3893.
We can currently specify a custom tooltip for an icon. This extends that to a generic tag [tooltip='something']...[/tooltip] for any particular piece of text, and removes the iconTooltip hardcoding by instead using a virtual GetTooltipText call - since all IGUIObject descendants have a tooltip. This is cleaner & more efficient.
I remove the ability to set a custom tooltip style (that doesn't seem particularly useful), it'll reuse the style of the CText itself.
I think this approach will be easier to extend if we ever want to add 'link' behaviour, by just adding a ButtonBehaviour and forwarding.