Construct a fuseView only if needed
Fix crash cases
Reset Undo redo stack after closing either the fixed or moving view.  fixes #1769
Remove a useless if condition and comment
Keep window level, avoid crash when removin fixed view after run an algo.
No more crash but window level not alwayssync between fuse and compare views.
Keep sync between fused and compare view even after removing one. Thx gpasquie.
Bug corrected: when removing a layer in the fuse view, the corresponding
view in the compare tab was not closed