FILEMAN-44: Add a checkbox on top of others to check them all

This is not good. Don't reset the selection when the user moves to a different page. The user must be able to execute batch operations (like copy or delete) on files that are displayed on different pages. I want to be able to select a file, move to the next page, select a second file and then click Copy to copy both files (so including the file that I selected on the previous page).