Improve user experience when dragging canvas

[playgithub]

Expected Behavior

When dragging the canvas with the right button down, the moving distance of canvas is equal to the moving distance of the mouse cursor.

Current Behavior

When moving the canvas by mouse dragging with the right button down, the moving distance of canvas is much longger than the moving distance of the mouse cursor.

Steps to Reproduce

1. Open a graph
2. Move the mouse cursor on the canvas
3. Dragging the canvas with the right button down
4. The canvas is moving much faster than the mouse cursor

Context

User experience is not good.

Your Environment

• Version used: Gephi 0.10.1
• Operating System: Windows, Manjaro