Node: fix whitespace (minor)
Refactor next branch node navigation shortcut

V now navigates to the next branch node or end of the largest descendant
branch of the active node, and subsequent presses of shift+V cycle
through other descending branches in order of descreasing size. Only the
initial V generates a request, while subsequent cycling uses the cached
response. This also allows some degree of local navigation before
cycling through sibling branches.

To enable this behavior, the API responds with an array of branches,
each branch containing a triple of child branch head, first node of
interest, and first branch node or leaf. Each of these nodes contains
the node ID and position. Redundant or unused location fetching helper
functions are factored into a single _find_locations function.

Includes new tests for this API view.

See discussion in acardona/CATMAID#834.
SVG Overlay: add shortcut to navigate to child

Add a shortcut key (]) to navigate to the child of active node. This key
behaves the same as V, i.e., navigates to the largest descendant branch,
with shift+] cyling through sibling branches in order of descending
size.

Also changes the shortcut to navigate to parent from P to [ for
consistency.

Closes acardona/CATMAID#834.