NEW "Show children as list" tree context action

Allows for easier navigation between tree and list,
particularly if a node has many children nodes
(since it doesn't require expanding that node).

Uses a new 'ss-tabs-force-active' class on the tabset
to enforce the correct view state. This also fixes an issue
where you couldn't link to a list view directly via URL.
Merge pull request #300 from chillu/pulls/show-children-as-list

NEW "Show children as list" tree context action