`mu4e-view-mode-map` was initialized from a sparse keymap and eventually passed
to `suppress-keymap` in order to remove bindings to `self-insert-command` which
obviously make no sense in read-only buffers. However, `suppress-keymap` also
remaps all digits and the minus sign to make prefix args, i.e., pressing 8
would be equal to `C-u 8` and `-` got bound to `negative-argument` instead of
`mu4e-view-mark-for-unflag` as defined.
While `suppress-keymap` has a `NODIGITS` arg to suppress the rebinding of
digits and minus sign (which would also mean that pressing a digit would try
inserting that digit and lead to an error), I chose the more straight-forward
solution and start from an empty rather than a sparse keymap and added the
`digit-argument` bindings for 0..9 explicitly.
34fe094561