Moving and modifying the Controller property editor

At some point we plan to move the Controller property editor to the Document panel.

1/ Rename Info section into Controller in Document panel
  - controller class property editor (with cog menu)
  - fx:root construct <CheckBox> (with informational ToolTip)
  - Assigned fx:id -> node read-only mapping

2/ Remove controller class property from Inspector

3/ Remove Edit > Use fx:root construct menu item

4/ Add menu item View > Show Controller (to be confirmed by Mo)


Notes about "deleting the root node"
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When the root node is deleted, document becomes empty (i.e. FXOMDocument.getFxomRoot() == null).
When document becomes empty:
  - controller class text field should be reset to "" and disabled
  - checkbox for fx:root should be unchecked and disabled.


"allow to control the Java class of the top element when loading the layout"
“Your Controller class must extend <root node class>”