Add template `pom-umbrella.xml`
Remove build plugin versions (moved to plugin-management in the parent)
Update the usage of `pallur_*` environment variables / Remove `--offline` for now
Major refactoring of parent POM

* Use the `pluginManagement` feature to specify the version of various build plugins in child modules.
* Update Apache Maven plugin versions.
* Reorder various XML blocks.
* Remove various obsolete XML blocks.
* Try to bind some common plugins to the lifecycle.  (Partialy achieved.)
* Try to configure if some plugins are disabled.  (Partialy achieved.)
Fix deploy script
Refactor parent POM to use profiles / Make build almost "garbage-less"
Implement full garbage-less build
Respect `TMPDIR` variable in run scripts
Minor fixup