Retain Library/Boards Manager installation interface using version menu

[francescospissu]

Motivation

If the version selected from the drop-down menu overlaps the next element, the focus on the current element is lost and the "INSTALL" button is no longer visible. Details here.

Change description

Keep the footer visible until installation is performed.

Other information

Closes #1425.

Reviewer checklist

• [ ] PR addresses a single concern.
• [ ] The PR has no duplicates (please search among the Pull Requests before creating one)
• [ ] PR title and description are properly filled.
• [ ] Docs have been added / updated (for bug fixes / features)