Make mouse wheel zoom more consistent with other actions and less confusing

[geometer]

1. For more consistency, save the zoom factor in the knot file between runs. I suggest dropping the separate "scaleFactor" field and use the existing transformation matrix instead.
2. Again, for consistency, the app should suggest saving of the scene if the user has scaled it. As far as I remember, changing of the matrix is enough to get this feature for free.
3. I suggest adding "zoom factor" field to the visual options dialogue.
4. As for "confusion", if I rotate wheel too fast, the scene becomes invisible, and I cannot even know if the knot is too small or too large. A possible solution is to set hard limits for the scale factor. Another idea is slow down zooming where the factor is close to the limits.

PS Probably, it makes sense to split this ticket.