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.

mlda065/gummi,turbopobre/gummi,alexandervdm/gummi,bobi32/gummi,alexandervdm/gummi,bobi32/gummi,mlda065/gummi,bobi32/gummi,turbopobre/gummi,bobi32/gummi,turbopobre/gummi,mlda065/gummi,mlda065/gummi,alexandervdm/gummi,turbopobre/gummi