Implement "lazy loading" in the generic "item overview"

[Nicklas Nordborg]

This means that a sub-tree should not be loaded until a user opens the parent node in the overview tree. To begin with only the root node and it's children should be displayed. To show more, the user has to open each child. The tree should open up one step at a time.

This ticket depends on #1245.

Lazy loading affects validation since we can't validate what has not been loaded. We need to complement the lazy loading with a possibility to update the validation information as more nodes are opened up.

This also affects the "Expand all" and "Collapse all" buttons in the overview. If we keep the buttons I think they should only use what has already been loaded. Eg. the "Expand all" button should not load the entire tree.

Lazy loading may not be appropriate when rooted at an experiment. This is because at this level there is often a need to validate the entire tree to make sure everything is set up properly. I think we may need a different solution for this case. It may be possible to start a background thread that does the loading/validation and show a progress bar... Hmm... maybe this should be a separate ticket...