The directions provided by itsme are spot on. If you've hidden the floating toolbar, go to Settings->Library Settings->Show Floating Toolbar to turn it back on, as that's where the select all button is.
EDIT:
As a side note, if it's more intuitive to long press a checkbox that is not checked to select all, and vice versa, I could add that feature easily.
EDIT:
As a side note, if it's more intuitive to long press a checkbox that is not checked to select all, and vice versa, I could add that feature easily.