Allow scan double side of single page if Flatbed is not found

With previous implementation the Scan Single Page
was working only in case the Flatbed is detected.
It seems that for ADF only scanners, scanning Single Page
could be also useful. Unfortunately simple-scan is scanning
only front page. With this commit it is possible to scan
Single Page with both sides on ADF only scanner.

Fixes #188
6 jobs for master in 18 minutes and 49 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #1266154
build-arch

00:03:35

passed #1266153
build-fedora

00:05:05

failed #1266151
allowed to fail
build:snap

00:08:41

passed #1266152
build-ubuntu

00:02:34

passed #1268283
flatpak
flatpak

00:10:08

failed #1266155
flatpak
flatpak

00:09:44

 
Name Stage Failure
failed
build:snap Test

subprojects/libhandy/src/meson.build:194:0: ERROR: Dependency 'gtk+-3.0' was already checked and was not found

A full log can be found at /builds/GNOME/simple-scan/parts/simple-scan/build/snapbuild/meson-logs/meson-log.txt
meson --prefix=/usr snapbuild
Failed to run 'meson --prefix=/usr snapbuild' for 'simple-scan': Exited with code 1.
Verify that the part is using the correct parameters and try again.
Cleaning up file based variables
ERROR: Job failed: exit code 1