core.module.thing(.ide): switch some files from .xtend to .java

[dilyanpalauzov]

This removes all .xtend files from the org.openhab.core.model.thing.ide bundle and removes the Xtend:compile target there.

Possibly from bundles/org.openhab.core.model.thing.ide/.classpath this can be removed, too:

        <classpathentry kind="src" output="target/classes" path="xtend-gen">
                <attributes>
                        <attribute name="optional" value="true"/>
                        <attribute name="maven.pomderived" value="true"/>
                </attributes>
        </classpathentry>

[wborn]

Correcting the previous review, which was truncated during submission.

AI performed an initial review before manual maintainer review.

The Xtend-to-Java conversions look behavior-preserving, and the Xtext generator configuration looks consistent with keeping Java stubs by default while retaining the existing Xtend validator.

The commit is missing the required Signed-off-by trailer. This PR changes source code and build/generator configuration, so the small patch exception does not apply. Please amend and sign off the commit before merge.

No other issues were found in the current diff beyond the inline .classpath cleanup comment.

[wborn]

Could the corresponding xtend-gen entry also be removed from org.openhab.core.model.thing.ide/.classpath? With the Xtend plugin and this source directory removed from the POM, the Eclipse classpath entry is now stale.