Add a dialog where config settings can be changed

[psavery]

This adds a dialog where the user can modify HEXRDGUI configuration settings.

Right now, the only option is to modify the number of processes used for multiprocessing.

The new menu item is under Edit->Configuration, and the dialog appears as the following:

We can likely add more configuration settings to this in the future, as needed.

Fixes: #1578

[bnmajor]

LGTM! My only question is if we want to remember the user's settings between sessions? It seems to me that if you need to limit resource usage for a machine you may want to limit it every time, but I also may be misunderstanding the use case here

[psavery]

LGTM! My only question is if we want to remember the user's settings between sessions? It seems to me that if you need to limit resource usage for a machine you may want to limit it every time, but I also may be misunderstanding the use case here

The user that requested this had mentioned that they start hexrdgui with a high number of CPUs on a shared machine, and then they want to reduce it later when they need to share the machine with others. So, they were specifically asking for a way to modify the number of CPUs in the middle of a session.

We can ask if we should remember this setting in between sessions. The potential problem, though, is that if a user sets it to something low in one session, they may forget about it in future sessions, and then they would not know why HEXRDGUI is performing slowly. Maybe a solution is to add a checkbox that says "Save as Default?" that will use it as the default. But we can ask the users first if this is something they want.