Scrollbars implementation

[lpickford-pixelbeard]

User Story:
As a user, I want the Gantt chart to display scrollbars when it is larger than the viewable area, so that I can easily navigate and view the entire project timeline.

Acceptance Criteria:

1. When the Gantt chart exceeds the dimensions of the viewable area, vertical and horizontal scrollbars should appear.
2. The vertical scrollbar should allow smooth scrolling up and down the Gantt chart.
3. The horizontal scrollbar should allow smooth scrolling left and right across the Gantt chart.
4. Scrollbars should only appear when the content of the Gantt chart overflows the visible area.
5. The appearance of the scrollbars should be styleable so that I can make them match the theme of my application.