Review IconBorder

The only two included changes noticeable by the user are:

• Use line number color for unfolded triangle to be less intrusive
• Move the line modification hint away from the edit area between the line numbers and the folding area to be less irritating when cursor is in first column

Most of the changes in this patch could you see as code cosmetic or an
attempt to make it more maintenance-friendly.

• Cleanup, avoid redundancies and nesting
• Rename members, make them unified named
• Give each area an own value name
• Try to fix bookmark pixmap painting, but that's still buggy
• Simplify "additional folding highlighting", there is now the slightly gradient gone, bad?
• Remove m_oldBackgroundColor, can't see any benefit
• Make positionToArea() and sizeHint() less error prone in case of design changes

Differential Revision: https://phabricator.kde.org/D20697