This is really just a workaround to keep SystemMonitor's process table
working right wrt selection retention during resorts (while also doing
full index invalidation on things like ProfileViewer inversion.)
It's starting to feel like the model abstraction is not super great
and we'll need a better approach if we want to actually build some more
dynamic functionality into our views.
This commit adds two new behaviour to the key event handler of
the TreeView widget:
Pressing left now jumps to the parent node if the current treenode
is closed or has no children.
Pressing right now jumps to the first children node if the current
treenode is open.
