Add option to switch off automatic model reloading

[hannobraun]

Automatic reloading of models doesn't work perfectly and might work differently between platforms. It would be good to have the option to disable it, if need be.

I'm not sure how best to do that though:

• A command-line parameter is an option, but that would require the user to always specify it.
• A UI element could be added (in which case this issue should be blocked on Evaluate options for improving GUI infrastructure  #4). That might be more convenient than a command-line parameter, in some cases, but any change to this setting would not be persist after Fornjot is restarted.
• Specifying it in a configuration file might be best, but we don't have that yet. (Also see Develop concept for configuration #38.)

I'm not sure if any of that is necessary right now, or if we should just wait until we have a more comprehensive configuration system. I don't have the need to disable reloading myself (and if I had, I'd rather fix whatever my issue is), so I don't know. Feedback would be very appreciated!

[hannobraun]

Adding https://github.com/hannobraun/Fornjot/labels/help%20wanted, since feedback on this issue would be appreciated.

[hannobraun]

Added an entry for this to the feature wishlist. Closing.