Remember last used system

[BrotherSharper]

The list of systems is quite huge and one game world would only ever use one system.
Please make it so the GM can set a system as default and also allow users to remember the last selected system.

[jsinme]

I've made the Roller open to the last used system instead of resetting to the top of the list.
For the GM setting, I've created a separate issue. #7