Upgrade the configuration system for gadgets

The configuration of each gadget is now managed with MobX. It means that we get rid of the mechanisms of FlexLayout and that we can configure items that are not displayed in FlexLayout.
Also, The configuration and readme mode are now displayed as a modal overlay over the whole web page. This is still WIP, but much better on some aspects than before.

Limitation : for now, if two tabs are instantiated with the same gadget inside, the configuration would be common to both. It clearly should be fixed.