Atlas Cinema Editor improvements

[elexis]

1. When adding a new cinema path in atlas, it would be more expected that the first look-At node is inserted near the place looked at too.

(Having the initial camera node position at the current camera location when later nodes are inserted at the camera look-at position is confusing.)

2. A path should always have at least 2 nodes.

A new path should be created with 2 nodes and the user shouldn't be able to delete path nodes if the path has only 2 of them. (A path with only one node is confusing, since the newcomer expects paths to be lines (which they always are unless starting a path).

3. Do not to use the current camera target as the coordinate for newly inserted nodes (except for the first two nodes) but:

3.a. extend the spline with the last angle (or extrapolated point on the spline) by some distance if it was the last node.

3.b. insert the node exactly between the selected node and it's successor node (if there is one) on the spline. (This should be a time-saver when editing since in most cases, the current camera position is optimized for editing the path with the node tool, not for imitating the path.)

4. Draw lines between each position node on the position spline and the according lookAt node. (This way the user can estimate the camera angle and align the paths without playing them)