This patch allows skipping the automatic insertion of nodes on top of
links when the transform operator ends. When putting nodes into small
spaces this often gets in the way and wastes time. Now, when holding
`alt`, this is turned off.
The header text is also improved to add this shortcut and to remove
the Dx and Dy values and improve the formatting a bit.
Making this functionality optional might allow us to use it in more
places in the future, like for the nodes added by link-drag-search.
Differential Revision: https://developer.blender.org/D16230
ae886596a0