Add ScrolledWindow around the project TreeView
Add ScrolledWindow around the project TreeView

This lets the right-hand pane become narrower than this TreeView.
Closes #417.

tonyolag/gummi-vim,jmarshallnz/gummi,jmarshallnz/gummi,tonyolag/gummi-vim,jmarshallnz/gummi,tonyolag/gummi-vim,jmarshallnz/gummi,tonyolag/gummi-vim