I get feedback when source or target attribute of <camunda:in/> element has trailing whitespace

[romansmirnov]

Context:

• BPMN Properties Panel

Expected behavior:

• Show a validation error message on whitespace in source or target
• Allow whitespace in source expression

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Cf. SUPPORT-3961

[nikku]

Why not simply remove trailing or leading whitespace?

Update: This is exactly what the customer suggests, too.

[nikku]

We've decided to validate instead of automatically trimming whitespace, as the later was too much magic.

[nikku]

Fails locally and on CI.

[nikku]

I've fixed the minor issue that made the stuff fail on CI.

Further change is needed though:

• We must not validate source expression, as it may contain whitespace: foo(a + b)
• We must validate source / target, these are variables that must not contain whitespace: abCD

I've updated the issue description accordingly.