Implements #112 thumbnails

applications/gui/CMakeLists.txt

applications/gui/src/gltexture.cpp

applications/gui/src/gltexture.hpp

applications/gui/src/media_panel.cpp

applications/gui/src/screen.cpp

applications/gui/src/src_window.cpp

applications/gui/src/src_window.hpp

applications/gui/src/thumbview.cpp

applications/gui/src/thumbview.hpp

applications/reconstruct/include/ftl/virtual_source.hpp

applications/reconstruct/src/virtual_source.cpp

components/rgbd-sources/include/ftl/rgbd/detail/source.hpp

components/rgbd-sources/include/ftl/rgbd/source.hpp

components/rgbd-sources/include/ftl/rgbd/streamer.hpp

components/rgbd-sources/src/bitrate_settings.hpp

components/rgbd-sources/src/image.hpp

components/rgbd-sources/src/middlebury_source.cpp

components/rgbd-sources/src/middlebury_source.hpp

components/rgbd-sources/src/net.cpp

components/rgbd-sources/src/net.hpp