#4720 - #4719 User Extensions - TOC plugins

[offtherailz]

Description

Created a first version of minimal User Extensions plugin with some changes on the TOC

• FilterLayer (for query panel), plugins as well as the others and modify TOC to be also a PluginContainer that receives the buttons for that functionalities. The existing AddGroup, FeatureGrid and LayerSettings should render a button as well
  Note: To avoid a refactor the toolbar buttons code is still in the TOC and the plugins as items don't render anything, simply enable/disable the button in the TOC. @mbarto what do you think? we should move the button renderings to the items at this stage ?

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:

Issue

What is the current behavior?

• First version of user extensions #4720
• Pluggable TOC to enable toolbar buttons #4719

What is the new behavior?

• First version of user extensions #4720
• Pluggable TOC to enable toolbar buttons #4719

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

Other useful information

[coveralls]

Coverage increased (+0.007%) to 84.524% when pulling 2f71f77 on offtherailz:user_extensions into d1e87d0 on geosolutions-it:master.