Adding mux GStreamer plugin libraries to Android (to enable video recording on Android)
Adding an option to limit the amount to disk used for saving videos
Adding an option to show/hide the REC button (allowing plugins to handle the recording in their own way)
Adding simple frame grab to file

GStreamer windows setup

Seems to work differently on appveyor build

* travis-ci install gstreamer

* update linux gstreamer dependencies

* fixes #2540

* travis-ci doxygen to trusty

While at it, fixed an issue with builds that have video streaming disabled.