Should not be able to enable if-return block outside of procedure definition

[vicng]

Problem statement

• Bug report
• Feature request

The "Enable block" context menu option should be disabled when the if-return block is not connected to a procedure definition block.

Steps to Reproduce

1. Add if-return block to workspace
2. Right-click the block. The "Enable block" option is selectable.

[samelhusseini]

More or less the same as #1390.

I think for this we should add a canBeDisabled property on a Blockly.Block, and only show the context menu if that property is true. The procedures_if_return block should set canBeDisabled to false.

[johnnesky]

Fixed by: #7958