Description of problem:
There is no update mechanism in Designer if collaborate two users on one process definition in the same time. User always rewrite the process by the his local definition. So win the last save.

Version-Release number of selected component (if applicable):
6.0.0 DR6

How reproducible:

Steps to Reproduce:
1. both user open process definition
2. user1: create process Start("MyStart") -> User Task("My User") -> End
3. user2: create process Start -> Task -> End
4. user1: save a process
5. user2: save a process
6. user1: delete End and add Task ("AAA") -> End
7. user2: change property "Name" of Task to "BBB"
8. user2: save a process
9. user1: save a process and close Designer
10: user1: open designer

Actual results:
The user1 can see a different process:
Start->Task("BBB")->End

Expected results:
Some notification about conflict:

• merging mechanism
• update designer if a process is saved
• etc.

Additional info:
GIT repository log says that commit "Updated asset BPMN2-ConditionalStart.bpmn2" is always done by same user (admin <null>).