Fieldmanager Gallery integration

[enrico-sorcinelli]

This PR aims to merge https://github.com/fusioneng/fieldmanager-gallery into FM.

The Fieldmanager Gallery provided is based on my PR that adds some fix and enhamcements to the latest Fieldmanager Gallery (that, to be honest, don't seems to be currently actively maintained and it still has a serious bug). In particular:

• Gallery disappearing. I merged these fixes that live in a branch (vip-fixed) that is some commits behind master.
• Added empty_gallery_button (only for collections) in order to remove a gallery. This could solve this issue
• Fixed the synchronization with the gallery preview and the modal media frame. Not too a serious bug but annoying.
• Uses fm_add_style and fm_add_script support functions to load JS/CSS
• The PR also contains PHPUnit test file for the gallery.

Note: There was two years old PR opened by @danielbachhuber but it does not seem to me that there have been actions over it.

[mboynes]

Thanks! I'll give this a thorough testing this week.

[enrico-sorcinelli]

I just updated the PR by adding a default value for the new "empty gallery" button label and I udpated relative test. Let me know if you need more in order to test this PR.