Create variable closes flyout if non-default dialog implementation is used

[microbit-matt-hillsdon]

At some point we fixed the plugin so creating a variable correctly leaves the flyout open.

However, we didn't account for the significant difference in behaviour between window.prompt and custom dialog implementations. It's still broken for the latter because custom dialogs take actual browser focus.

For custom dialogs, focus will need to move to the dialog for input and return to the flyout afterwards. In the meantime we'll need to know to keep the flyout open. Blockly will have to be responsible for at least the latter part and it might be prudent to do restoring focus too rather than trust the dialog implementation.

It might be good to add a test scenario that plugs in some simple <dialog>-based dialogs.

Potentially relevant to @BenHenning's focus work.

CC @microbit-robert