pat-filemanager UX bugs and a11y

Original discussion https://community.plone.org/t/meet-pat-filemanager-a-plone-blicca-successor-to-pat-structure-folder-contents/23063/4

• If you select two items and click on the "2 selected" button, it unchecks the checkboxes for those two items, but the "Select All" button itself remains checked.
• Breadcrumbs - the home icon link does not work as expected if you access the folder contents from a folder. Example: Go to /plone/folderA/folderB/, go to folder contents, click Home icon. I would expect this to go to /plone but it goes to /plone/folderA/folderB
• The rearrange/sort button adds a new status message every time as opposed to pat-structure where it replaced the current message
• Modal forms - this uses the placeholder attribute on some inputs. Since the text presents instructions to the user, we think it would be better as help text below/above the input. It's also worth noting that default Plone does not use placeholder in any forms

a11y:

• On focus, the grid/listing toggle has no visual indicator on Firefox only.
• Our a11y person flagged the grid view because visually you get an indication of content type via the icon, but this isn't presented by a screen reader. My interpretation is that you aren't really trying to convey content type here, those icons are just filler if there is no preview image. There's a separate column available to show content type. Maybe have an aria-label for type anyway?

Start at the pat-filemanager implementation and reproduce each listed behavior: selection state, breadcrumb Home navigation, rearrange status messages, modal help text, Firefox focus indication, and screen-reader content-type information. Done means the reported UX and accessibility issues are resolved without regressions across the affected views.