Replace hard-coded handlers with lookups

Currently, handlers for cell types are hard-coded into switch statements. This reduces flexibility to introduce plugins. We need a completely new design pattern.

Rather than hard-coding, handlers need to be put into a hash table with a mapping to their cell-type. Plugins can then append to this hashtable to add their own handlers for their own cell-types.

Needless to say, cell-types will also need this flexibility so that new cell-types can be added on the fly.