Add support for terminal like caret

[PavelTurk]

RichTextFX is also used for building terminal emulator applications. At the same time, currently it is not possible to style caret to look like in terminals. This problem was also discussed in https://github.com/FXMisc/RichTextFX/issues/397 , but later the issue was closed without resolving.

I suggest to support the following caret features:

1. width
2. height
3. shape
4. color
5. inverse color (for example, the color of the letter that the caret is on)
6. caret position (it is necessary to set caret over letter)

[Jugen]

PRs are welcome for new features :-)