CoreText - Setting styles on empty AttributedStrings

[build 0058] Text in Cello appears to be completed - for the moment at least.

The last problem with text was something of a classic. Within Cello the text and style runs are stored as CFAttributedStrings. This works out really well as this is what CoreText consumes. The problem comes with empty strings. If you have an empty string then it is impossible to set any attributes on it. Also if you try and find the bounds of the string CoreText will give you an empty rectangle. This is not necessary what is wanted. If Cello is calculating the height of a text box then if text styling has been set then it needs to be able to set the styling of the text box and to use this to calculate the height of the box.

The solution to this problem is to introduce the idea of the NULL style. The NULL style is a style that is set and exists when there is a need to set the style at a point rather than over a selection of text. So, when there is no text, setting the style of a text flow will just set the NULL style. Then when text is actually added we grab the NULL style and style the newly added text with it. A similar concept can be used when you set the style at an insertion point.

As far as getting the height of text when thee is none - a solution that works is to give CoreText a single space character and to style it with the NULL style. Then the normal code that is used to calculate the box height will just work.