Bubble menu: Disable and show cross if statement form is active

[Betree]

When clicking on "add statement" then scrolling down, some stuff can be confusing: autoscroll is disabled, and user may not understand why. He may not be able to remember he had the form open in the first place.

We can improve this experience by making the bubble menu grey and showing a close icon if statement form is currently active:

Clicking on this cross would close the statement.

[Betree]

Bubble menu: Disable + cross if statement form is active

[Betree]

Done with #44