Clattered tree view

[lior-antonov]

The left panel now feels a bit clattered with all the features together.

I think we need to keep the tree view on the top of the panel always and have two smaller "sub-panels" at the bottom one for notebooks and the other for repositories:

**************************
                                                   
        Tree View                          
           ...                                     
                                                   
                                                   
                                                 
**************************
    Repos   *  Notebooks  
      ...   *    ...                 
            *                        
**************************