Generalize icon-specific tooltips to a tag, streamline code.

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 both cleaner & more efficient.
Removes the ability to set a custom tooltip style (that doesn't seem particularly useful), it'll reuse the style of the CText itself.

Differential Revision: https://code.wildfiregames.com/D3895