Enable the Settings tab

[krikru]

In the Settings tab, It should be possible to:

1. Set the system settings via the various input widgets that exist in the tab. All values should be converted to the right units when transferred from the form to the system settings.
2. Save/load the settings to/from an settings file chosen by the user. When saving the settings, the units should be saved along with the values instead of converting the values to other units.

The fields should of course also already be loaded when the application starts.