Don't trigger preferences_saved so often

[pento]

The preferences dialog triggers the preferences_saved every time a preference input value changes.

This is a problem for options that might need several changes. For example, changing the port from 9999 to 9998:

• Click in the box
• Press backspace, changing the value to 999, triggering preferences_saved
• Type 8, changing the value to 9998, triggering preferences_saved again

A couple of options worth thinking about:

• For preferences that would cause a high load, add an "Apply and Restart" button. Docker uses this.
• Wait for the input element to lose focus before triggering the event, or wait until the preferences window closes.

[pento]

36 fixed this by only triggering the update when the element loses focus.