webglapp: destroyable mouse controls that don't retain a context
that would capture elements of the 3d view like Space etc.
neuronstagingarea: prevent error when closing it if the 3d view is not open.
webglapp.js and neuronstagingarea.js now "use strict";
webglapp.js: dispose of geometries and materials when done with them.

The 3d widget had never done that, surprisingly.

Additionally, now all Skeleton instances reuse the connector materials,
given that their color never changes.
neuronstagingarea.js: fix inability to remove first element of the selection.

Turns out that zero is falsy in javascript.
Merge branch 'master' of github.com:acardona/CATMAID

Conflicts:
	django/applications/catmaid/static/webglapp.js