Don't expand Timegaph trees by default

[bhufmann]

When opening a time graph view the tree is fully expanded by default. This might be ok for some of such views, but not for all. For example the Resources Status view of the Trace Compass server show the CPU states and it's children by default. However, it should only show the first level and on user demand the children of should be shown.

Since this behaviour is not the same across the different time graph views, expansion level of the tree needs to be provided by the corresponding data provider back-end.

The solution could be similar to Eclipse Trace Compass which provides an API so that adopters can define an expansion level for the different time graph views. However, the solution here can be different.

This is how the Resources View looks like with code on master branch of today after opening it:

For this view all nodes should be collapsed instead: