refac(labels): Renaming of css getter function
fix(css): Gantt border is shown far to the right if content is smaller than gantt width

If the content is smaller than the gantt width the border is still displayed on the far right resulting in a ugly gap. This can only happen if columnWidth if set and the content is smaller => e.g. when the data range is small and viewScale is quarter/year.
fix(options): Boolean default options are not handled correctly

Boolean default options which are true are returned as undefined. Because if this an issue occurs with the side section. The side section has now a fixed defined height to simplify gantt resize calculations. Thus if no side plugin is loaded the side section should be hidden (default) otherwise its shown as empty.