#4732 Fix MapFooter update issue

[offtherailz]

Description

The issue #4732 was due to a shouldComponentUpdate that always return false. This to prevent attribution and scalebar to disappear.

The solutions proposed keeps the divs to restore them after re-mount, because it's impossible to avoid re-mount.

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

Issue

What is the current behavior?
#4732

What is the new behavior?
Now a context with optional plugins in footer will update immedialy, when user switches them in user extensions plugins (need to be merged yet)

Breaking change

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

• Yes, and I documented them in migration notes
• No

--> we need to update localConfig.json in dependent projects.

Other useful information

Probably old context will not render attribution well

[coveralls]

Coverage increased (+0.007%) to 84.511% when pulling 5fa7861 on offtherailz:fix_footer_pluggability into 9db5b72 on geosolutions-it:master.