Drop Mozilla plugin

Closes #1340

See merge request !22

It is no longer supported by Firefox
or any other major web browser.

Closes: #1340

No need to bloat tarballs with unneeded files

File monitors don't work, but async file enumerators do.

Previously this was only needed for Samsung phones, but it appears
that starting with Android 9 (Pie) this is necessary for new media
files to be added to the media library.

The attribute map also needs to be initialised.  Might fix
#1660

gvfs currently does not invalidate its mount info cache when a mount
is removed unless it has a connection open to the mount process, so
when another mount appears at the same path, it uses stale
information to talk to the mount process, so all operations fail.

We can work around this by monitoring the root of the mount, which
causes gvfs to keep a connection open.

audioscrobbler: Don't break https:// URLS

See merge request !21

These days last.fm APIs return us https:// URLs, however, the code to
open these links was turning them into http://https:// URLs. This
resulted in them not working at all.

Signed-off-by: Trần Ngọc Quân <vnwildman@gmail.com>