Rename base source class

applications/reconstruct/include/ftl/virtual_source.hpp

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

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

components/rgbd-sources/src/source.cpp

components/rgbd-sources/src/sources/image/image.hpp

...rgbd-sources/src/sources/middlebury/middlebury_source.cpp

...rgbd-sources/src/sources/middlebury/middlebury_source.hpp

components/rgbd-sources/src/sources/pylon/pylon.cpp

components/rgbd-sources/src/sources/pylon/pylon.hpp

...s/rgbd-sources/src/sources/realsense/realsense_source.cpp

...s/rgbd-sources/src/sources/realsense/realsense_source.hpp

.../rgbd-sources/src/sources/screencapture/screencapture.cpp

.../rgbd-sources/src/sources/screencapture/screencapture.hpp

...nts/rgbd-sources/src/sources/snapshot/snapshot_source.cpp

...nts/rgbd-sources/src/sources/snapshot/snapshot_source.hpp

...ents/rgbd-sources/src/sources/stereovideo/stereovideo.cpp

...ents/rgbd-sources/src/sources/stereovideo/stereovideo.hpp

components/rgbd-sources/test/source_unit.cpp