Lift restrictions of zoom in 3D view to allow closer views

[ch-schiffer]

The zooming level of the 3D view is quite limited at the moment. For example, it is not possible to get a close-up view of subcortical regions (e.g., LGB, MGB). The restriction should be lifted, so users can zoom in as far as they like.

[xgui3783]

The original intention of locking 3D view is that the 3D view provides rather poor resolution in cross sections, and that it was meant for a rough guide where the navigation is at (relative to the whole brain).

Most recently discussion touches on two relevant behaviours we are working towards:

• locking the 3D view completely. zooming in 3D view will result in the zoom'ing of slice view instead
• remove the slice view in 3D view

Initially, we certainly did not have high resolution meshes we would like to display. I can bring this issue up in the next dev meeting.

[xgui3783]

related to #1138 ?

edit: and #1137 ?