Customize the default CustomizableUI toolbar using CustomizableUI.jsm

Currently we set a non-empty browser.uiCustomization.state preference to control adding buttons to the toolbar. This is just a large JSON of the button placements.

However, browser.uiCustomization.state always changes to a different non-default value after first loading the application because the current toolbar state gets saved on top. As such, we cannot add or remove buttons using this preference and have it also effect existing users.

Instead, we should do what firefox does and update the toolbar state using a soft version number.