Serialization fixes and an extra DiscreteAction

I had a problem with flybywire not saving my manual deadzone settings, which I have now fixed. I added a DiscreteAction for doing pitch trimming, which works really well compared to analog trimming. I also moved the toolbar button image into the addon folder, since this is what other addons seem to do. I lost a lot of settings because I didn't know I had to manually save configuration under Mod Settings first, so I made it save state whenever a setting window is closed, which might help a lot of new users.

This is my first ever pull request on a KSP addon, so do inspect my code and make sure I didn't make any mistakes. Thanks for a really awesome addon!