Fix Maven compile panel to respect Maven default JDK

[NicolaIsotta]

When the Maven default JDK differed from the IDE default JDK, the compile panel's Java Platform combo-box would show the IDE default, while Maven actually used its own configured default to build. This could be misleading to the user e might caused builds to fail.

The combo-box now correctly reflects the Maven default JDK and labels it as "(Maven default)" when it differs from the IDE default.

Fixes #3928

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[NicolaIsotta]

Steps to test/reproduce can be adapted from #3928

For reference, here's the current behavior:

Here's how the panel appears with the PR applied: