Added left/right key navigation in solution viewer.
lluis suggested I should base it on cases. As in, only allow left/right key navigation where only one node is selected.

Left key on file navigates you to parent folder. Left key on folder while collapsed, navigates to its parent folder/project.
Right key on folder navigates to its first child if expanded.
Code cleanup.
Fixup that slluis asked for.
Merge pull request #268 from Therzok/keyNavigation

Added left/right key navigation in solution viewer.