Refactor names in `URTFUtils` unit

[delphidabbler]

This unit has several RTF*** routines that could do with grouping together in a TRTF record w/ methods.

But, TRTF is currently used to encapsulate RTF markup. So rename TRTF as TRTFMarkup or TRTFCode.

The unit also contains TRichEditHelper which contains a single method to help loading RTF code into a RichEdit control. This doesn't sit well here. Perhaps it would be better if moved to its own unit, and maybe converted into a class helper.