subparse: Autodetect subtitle text encoding

Use "uchardet" to guess the subtitle text encoding if it is not in UTF-8
or manually specified instead of blindly guessing its "ISO-8859-15".
The "uchardet" dependency is optional and when is not available at
compile time, then old behaviour will be used.
34 jobs for !942 with subs-autodetect in 25 seconds (queued for 5 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #5802079
alpine amd64 manifest builder docker

00:00:14

passed #5802078
android docker

00:00:14

passed #5802077
cerbero fedora amd64 docker

00:00:13

passed #5802076
fedora amd64 docker

00:00:15

passed #5802080
gst-indent amd64 docker

00:00:12

 
  Preparation
passed #5802082
gst indent

00:00:10

manual #5802081
manual
manifest
 
  Build
created #5802091
gstreamer
build cerbero cross-android universal
created #5802095
gst-ios-13.2
build cerbero cross-ios universal
created #5802092
gstreamer
build cerbero cross win32
created #5802093
gstreamer
build cerbero cross win64
created #5802090
gstreamer
build cerbero fedora x86_64
created #5802094
gst-macos-10.15
build cerbero macos x86_64
created #5802096
1809 gstreamer-windows windows docker
build cerbero msvc x86_64
created #5802086
build clang fedora x86_64
created #5802089
1809 windows docker allowed to fail manual
build msys2
created #5802083
build nodebug fedora x86_64
created #5802084
build static fedora x86_64
created #5802085
build static nodebug fedora x86_64
created #5802087
1809 windows docker
build vs2017 amd64
created #5802088
1809 windows docker
build vs2017 x86
 
  Test
created #5802097
gstreamer
check fedora
created #5802098
gstreamer
integration testsuites fedora 1/4
created #5802099
gstreamer
integration testsuites fedora 2/4
created #5802100
gstreamer
integration testsuites fedora 3/4
created #5802101
gstreamer
integration testsuites fedora 4/4
created #5802105
gstreamer
valgrind bad
created #5802102
gstreamer
valgrind base
created #5802106
gstreamer
valgrind ges
created #5802103
gstreamer
valgrind good
created #5802104
gstreamer
valgrind ugly
 
  Integrate
created #5802107
cross-android universal examples
created #5802109
gst-ios-13.2
cross-ios universal examples
created #5802108
documentation