Add widgets to the dashboard, live!

See merge request behaviors-ai/the_dashboard!11

This required to alter the way the modal dialog is handled.
It formerly was only used for the info and settings of a widget but now is much more generic.

Allow gadgets to have their own configuration

See merge request behaviors-ai/the_dashboard!9

Dgoepp/fixes

See merge request behaviors-ai/the_dashboard!10

fix a tiny bug in Video Stream

See merge request behaviors-ai/the_dashboard!8

remove test code for the Graph gadget

See merge request behaviors-ai/the_dashboard!7

Make all gadgets configurable

See merge request behaviors-ai/the_dashboard!2

Finally, a graph visualisation

See merge request behaviors-ai/the_dashboard!6

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.

The data of the interaction trace object were stored in the global MobX store with the key "interactionTrace". To prepare for the further work, I changed it to match the name of this object as it is known to FlexLayout.

Dgoepp/fixes#1

See merge request behaviors-ai/the_dashboard!5