Create CELineHeightTransformer class (ref #177)

to convert line spacing <-> line height
Make line spacing setting UI line height based (close #177)

Also including the following changes:
- change line height field in preferences from combo box to normal text field + stepper
- change default line spacing from 0.0 to 0.3
- fix custom line height panel behavior
Merge branch 'refs/heads/feature/issue177_lineHeight' into develop