fix some working set view issues

Fixes #9019 
Fixes #8973
Fixes #9032

WRT #3, I think this is correct

1. Open 2 files (editor.js in one pane, mainviewmanager.js in another)
2. Click on a file that isn't open anywhere (pane.js)
--> pane.js and editor.js are both blue and both visible in panes.  mvm is hidden.
3. Click editor.js in file tree
--> editor.js is the only file that is blue (it's blue in both the project tree and workingsetview) 

This is because pane.js is open in the other pane which isn't in the working set.  
I can see how that may lead to some confusion as pane.js isn't blue anywhere because file tree doesn't allow for more than one selected item.

I wonder if @dangoor's new file tree would allow for this and if it would even make sense. I guess it could be blue and not "selected" but when we allow for the same document to be open in 2 different panes would that even make sense?