ProcessTreeView: clean up

[danirabbit]

• Gobject-style construction
• Name doesn't need to be bound because name and icon never change in the source object
• Name doesn't need an unbind because its values are only set once in bind
• DRY label unbind
• Don't need to manually set expand = false on non-expanded columns
• Don't need to hexpand name labels because the column is set to expand and labels fill by default
• Rename functions to start with the thing they do (setup_, bind_, unbind_)
• Organize and clean up extra variables