Major re-vamp of the visualisation code: caching and tooltips

Instead of generating the images on-demand every time they're asked
for, add a cache and generate them when the index is asked for. Only
update the cache if the database underneath us has changed. This will
reduce load noticeably when nothing has changed, plus in the
index-generating code we now have access to information about the
images.

We now use that information to generate pop-up tooltips when the user
hovers over each port in the network graphics, giving more information
about the port in question. Major modifications to the CSS needed to
make this work too. Tested successfully in Firefox and Chrome so far;
IE and webkit are not showing the tooltips yet. :-(

Change-Id: I7d2f3ddf289c0ca4379a8e287c37e2e043745487
1 file changed