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 and might cause 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: