Running PersefoneDesktop with this custom system image, latencies are
reduced as follows:

- startup time of the GUI (until the map is shown): 50s (previously 95s)

- showing the population graph window: <1s (previously multiple seconds)

The system image is created with `sysimg_create.jl`.

To use the system image, run `julia -J sysimg.so desktop.jl`.

The majority of the remaining startup time (45s of 50s) is due to the
initialisation of the Persefone simulation.  This might be reduced by
further improving persefone-model.