Gimp-like scrolling

This issue is a following to issue #33 (closed)

It would be great if the way scrolling is done is like the one in Gimp. Moving through scroll bar:

• At 100% zoom the scroll bar take the entire range, it can't be moved
• At (X * 100)% zoom the scroll bar take 1/X of the range and can be moved smoothly

Moving through mouse button:

• The image can be moved until it covers half of the screen for each axis
• Thus moving can be done on a longer range than with the scroll bar only
• When moving out of range the scroll bars reduce in size