Improve bar line Inspector and Palette

**Inspector**
- Align span combo item names with palette names
- Add a [Custom] span item: it is informative only, cannot be selected and is set by the Inspector for spans which do not match any built-in span
- Implement proper selection of span combo item when a bar line is selected in the score
- Enable/disable type combo items according to bar line type (repeats cannot be turned on/off for individual bar lines)
- Implement spanTypeActivated() to set in the score element the span selected in the Inspector

**Palette**
- Labels for styles and spans made clearer (hopefully...)