Multi-Select when importing Books

[iNewmanity]

Is your feature request related to a problem? Please describe.
When I import my Books i only select one at a time. There is no possibility to select multiple ones in the file-browser

Describe the solution you'd like
Enable multi-select in the file browser.

[IBBoard]

There's already a ticket for this with a command-line workaround - #1196 (comment)

[Jopp-gh]

Opening single PDFs in a eBook reader is simply impractical.

Instead to add a "multiple file option", I'd prefer if Foliate could open a folder chooser to import recursively all PDFs / ebooks from a selected folder.

To go a step further, Foliate could "watch a folder" and add new books automatically.
Such features would be adequate for the quality standard Foliate strives for.
Watching a folder for changes is nothing special or incredible difficult to implement.