* travis-ci install gstreamer

* update linux gstreamer dependencies

* fixes #2540

* travis-ci doxygen to trusty

This reverts commit 02f69bea.

Allows QGC to run on Raspberry Pi 2 with direct framebuffer (X/startx
not needed). Memory split seems to need 128 for GPU. Qt5 needs to be
configured with "-opengl es2 -qpa eglfs -device linux-rasp-pi2-g++".

There are many ways of iterating over lists in C++/Qt.
In the specific case of QStrings, it is known [1] that using
foreach with const references avoids the creation of new
QString objects in each iteration, reducing the time consumed
by the loops.

[1] https://blog.qt.io/blog/2009/01/23/iterating-efficiently/Signed-off-by: Anselmo L. S. Melo <anselmo.melo@intel.com>

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