Conditional Closing is bypassed when middle-clicking with the mouse

[Jacques Bourque]

Current behavior
Using the middle button of the mouse to close a tab bypasses the conditional closing of a tab

Expected behavior:
The behavior should be the same as clicking the X button on the tab

Example:
http://golden-layout.com/examples/#d66d6ee7794a8e000d72e9aa51c9bb9c