Revert "brings back window_thread"

This reverts commit 41a655e8e99910c13a3e7f1ebfdd083207cef76f.
Revert "make sure waitKey is called after imshow"

This reverts commit d13e3ed6af819459bca221ece779964a74beefac.
Revert "remove GTK dependency"

This reverts commit a6e15e796a40385fbbf8da05966aa47d179dcb46.

Conflicts:
	image_view/CMakeLists.txt
	image_view/src/nodelets/disparity_nodelet.cpp
	image_view/src/nodes/stereo_view.cpp