Add back-end interface to get menu information for stacks of a project
Retrieve stack menu info when needed instead of caching

In three spots, the stack menu information for the current project was
needed. Up to now cached information was used to get this data. This
commit changes those spots to request the stack info right when needed.
This simplifies the code a bit as the cache doesn't need to be kept up
to date anymore.
SVGOverlay: remove mouse event functions from node and connector
when obliterating them.
Remove not-needed project- and stack-data cache

This cache is not used anymore and can therefore safely be removed.
Fixed numerous accidental global variables.
SVGOverlay: connector arrows are now cached.
Caching avoids removing and inserting into the DOM, an expensive operation.
Merge remote-tracking branch 'origin/remove-project-data-cache'

Conflicts:
	django/applications/catmaid/static/init.js
Fix typo in comment.
ArrowLine: a single mousedown function for all arrows.
Fix accidental global variable.
SVGOverlay: fixes for connector edges.
SVGOverlay: more fixes for connector arrows.

The main issue is with the connectorDrawEdges having two functions:
(1) creating the arrows and (2) updating them when moving a connector.
better help icon
better disclaimer icon
Clustering: use SciPy to do the actual clustering
Add gRaphael line graph v0.5.1 library
Add world/screen coordinate translation methot to gRaphael line chart
Add min/max axis options to gRaphael line chart
Clustering: draw a basic dendrogram
Clustering: show cluster names when hovering the dendrogram