details-page: Add screenshot viewer

Currently there is no option to make the screenshots larger which often
makes them hard to read.

To fix this situation, open a simple screenshot viewer after
clicking on the main screenshot. It 'zooms' in, making the screenshot full screen
with left/right arrows to flip between screenshots, and an X to close it
and return to the application view.

Closes GNOME/gnome-software#313
Status Job ID Name Coverage
  Build
failed #655250
debian-stable-x86_64

00:01:27

passed #655249
fedora-x86_64

00:03:10

 
Name Stage Failure
failed
debian-stable-x86_64 Build
Uploading artifacts...
_build/config.h: found 1 matching files
_build/meson-logs: found 4 matching files

_build/debian-stable-x86_64-report.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=655250 responseStatus=201 Created token=YRwLA7jd
Uploading artifacts...
_build/debian-stable-x86_64-report.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=655250 responseStatus=201 Created token=YRwLA7jd
ERROR: Job failed: exit code 1