Remember last used camera

During setup_camera() in cheese-application.vala, we were not explicitly
assigning the camera from settings and only assigning the camera from
the command line.

This now checks for values from both the command line and from settings
and allows the command line to override settings.

Fixes: #76
1 job for master in 6 minutes and 17 seconds
Status Job ID Name Coverage
  Test
passed #909416
build-fedora

00:06:17