Add additional keyboard navigation: Ctrl+Up & Ctrl+Down (under the "Go"

Add additional keyboard navigation: Ctrl+Up & Ctrl+Down (under the "Go"
menu) move to the previous and next packet even if the packet list isn't
in focus (specifically, the packet detail will stay in focus if that was
already the case).  Make the return and enter keys toggle tree items in
the packet detail and the backspace key jump to the parent node.  Add a
navigation section to the User's Guide.

Make SIGNAL_EMIT_BY_NAME a simple #define, so that we can feed it
multiple arguments.  Fix up whitespace.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@19924 f5534014-38df-0310-8fa8-9805f1628bb7