Remove useless option
Attach tooltip to the character rectangle rather than the small arrow

This allows to show tooltips with mouseover on a violation character.
Tweak violation arrow and tooltip position
Copy & paste Tooltip::show for preparation of customization
Show tooltip above the violation when there's no space below it

Related to #12.
Copy & paste Tooltip::applyPlacement and ::setContent for customization
Place tooltip smartly according to the violation position in the editor

Closes #12.
Merge branch 'smart-tooltip-position'