[2.0.0-alpha1] Form builder: Simpler sources selector (1/2)

Not finished yet.

Current work:

• default source in sources selector
• form plugin AJAX update
• structure of the submitted data
• width fitting in layout builder sidepanel

Current work:

• default source in sources selector >> OK
• form plugin AJAX update
• structure of the submitted data
• width fitting in layout builder sidepanel >> OK

Remaining work on props:

• structure of the submitted data (not saved)

Remaining work on slots:

• Remove UiPatternsOperations form element

Also:

• Remove SourcePluginManager::buildPluginConfiguration()
• replace self by static?
• $wrapper_id = 'ui-patterns-prop-item-' . $prop_id; > ADD random if many form

Ready to be reviewed

Automatically closed - issue fixed for 2 weeks with no activity.