Selecting and editing shapes and view change issues

There was a crash after changing the view (View tab -> any of the orientation options) because there were some operations on the DocumentNodeBuilder which is this case doesn't exist so a null reference exception was thrown. Added the necessary checks and the view is changed correctly and editing/selection can continue.

A second issue was selecting the shapes, because the EditingHandlers for some shapes didn't use the new structure where references were passed. After selection started working, I tested editing the shapes using the handles that appear after selection and fixed the crashes.

Editing for arcs is possible using either of the three points:
For the Center, Start, End arcs dragging one of the arc margin points (Start or End) allows us to redefine the size of the arc by moving the point on the contour of the 'circle'. Dragging the Center allows us to change the radius of the arc. There is a problem when previewing this, because the arc keeps flickering and getting really big radius changes in the same point, but I'm working on it.

For the Start, End, Radius arcs dragging one of the margins (Start or End) changes that margin's position (this time it's not done only on the circle contour). After the distance between these points is the desired one, moving the third point sets the new radius.

When editing the shapes, a control appears in its middle, with the planes and axes. These are not done on the same scale as the shape, but as the main axis system, so if the view is zoomed they becomes too big or too small. Also, clicking/dragging it causes the shape to behave in strange ways. Same zooming problem with selection markers and the dimension arrows.

The spline tools work ok, with a small problem for the interpolated spline and spline control points: after drawing and selecting the spline, its line becomes white and is difficult be seen (hovering over it turns it red, so it can be selected). The Split spline and Spline Combine actions work well (the split/combine points are visible).

The changes are here. For the next revision I'll try to fix the issues I've mentioned here:
* CSE arc resize
* edit controls scaling
* edit axis system behaviour
* spline line color