Diagram accepts dropped elements even if in read only.

[nvannr]

• I have checked that this bug has not yet been reported by someone else
• I have checked that this bug appears on Chrome
• I have specified the version :

• latest

• I have specified my environment :

• Windows

Screenshots

Steps to reproduce

• Drop an element in a read only diagram

Expected behavior

Drops shouldn't be allowed in read only diagrams.

Actual behavior

Drops are allowed in read only diagrams.