Do not reset viewpoint when changing tabs or program

[TomNysWF]

Is your feature request related to a problem? Please describe. When changing tabs to another tree and comming back to the previous tree, the viewpoint for this tree is reset to the default, if we have zoomed in somewhere, it will always zoom back out, this is a real problem when dealing with bigger trees.

Describe the solution you'd like Keep zoom of the tab when switching tabs