Fix highlight fill

[Jugen]

The highlight fill property has been broken since 0.9.0 as a result of the multiple selection capability being added.

It can now be set again via CSS with either of:

.styled-text-area .selection { -fx-fill: lime; }
/* or */
.styled-text-area .selection { -fx-highlight-fill: lime; }
/* or */
.styled-text-area .main-selection { -fx-highlight-fill: pink; }
/* or */
.styled-text-area .main-selection { -fx-fill: pink; }