WIP: List/Tree View + CellRenderer

@vtjnash: I am making this a WIP issue again, so that the developement is tracked here. Feel free to merge from time to time.

Some random notes:
 - I needed a `gtypes` function for the ListStore constructor. Its implementation may not be the best but the code is now working
 - I have now implemented different CellRenderers
 - I have put an example into the test.jl file. But this will need some actual `@test` calls
 - The TreeView does currently crash. Not sure why