gesdemux: Detect recursively loading the same project file

And error out when it is the case.
16 jobs for master in 54 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #403316
gst indent

00:00:18

passed #403315
manifest

00:00:57

 
  Build
passed #403322
build cerbero android universal

00:14:24

passed #403323
build cerbero cross win32

00:13:15

passed #403324
build cerbero cross win64

00:07:21

passed #403321
build cerbero fedora x86_64

00:10:00

manual #403326
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #403325
gst-macos-10.14
build cerbero macos x86_64

00:08:26

passed #403317
build fedora x86_64

00:10:30

passed #403318
build nodebug fedora x86_64

00:09:05

passed #403319
build static fedora x86_64

00:17:22

passed #403320
build static nodebug fedora x86_64

00:13:23

 
  Test
passed #403327
check fedora

00:03:27

passed #403328
integration testsuites fedora

00:29:28

 
  Integrate
passed #403329
android universal examples

00:06:44

passed #403330
documentation

00:05:03