Implemented a resource manager for loading resources in parallel with
loading models during application startup. 
The resource manager only takes up a single thread.
The api to the resource manager is very simple:
One method to register and associate Callable<T>'s which provide
resources which unique names for the particular resource.
- and one method to retrieve a resource using those unique names.

Currently, this is used to load JChooserChooser's. These were previously
loaded in a lazy sense which implicated a slow loading of the shown
popup menu after application startup. This effect does not occur now (on
a machine with 2 cores, each with two logical threads).

Signed-off-by: Jakob Brandsborg Ehmsen <jakobehmsen.github@gmail.com>