Fixes #2526: download button in the TOC

[mbarto]

Description

Add a shortcut in the TOC to download layer's data through WFS

Issues

• See title

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)
You can download layer's data only from the featuregrid.

What is the new behavior?
You can download layer's data directly from the TOC.

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 increased (+0.01%) to 80.99% when pulling adf7f06 on mbarto:toc_download into 9ec8b5a on geosolutions-it:master.