Enable fetching categories from visualization.

Often tab `update` events are not triggered (eg. reopening tabs from
previous session?) and so we don't fetch categories for them. This
provides a button to enable the user to fetch categories for these tabs
from the visualization.

Some architectural changes were required to accomplish this, mainly
setting up a chain of callback accepting functions, starting from `getCategory()`.
Switch to tab directly from tree nodes.

This provides an icon next to each node in the tree (the unicode version
of >>) to switch to that tab.
Add help file and style popup.
Update README.
Update README title.
New icon
New icon
Merge branch 'master' of https://github.com/ameyp20191/hccProject
Show group by 'Website' instead of 'URL'.