Some more fixes for the plugins example: themes and config editing

[mbarto]

Description

Plugins example still needs some fixes to the config editing

Issues

• Fixes plugins example config editing

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/geosolutions-it/MapStore2/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x", remove the others)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
Config help not working

What is the new behavior?
Config help working

Does this PR introduce a breaking change? (check one with "x", remove the other)

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[coveralls]

Coverage remained the same at 80.686% when pulling 77ac473 on mbarto:more_plugins_example_fixes into 8e9f08d on geosolutions-it:master.