Merge pull request #156 from ted-n/gstreamer-samples

gstreamer: sample stepping

Patch by Takashi Nakajima. Thanks!!!
gtk3: guard inserted cell by TreeView#insert_column from GC

[ruby-gnome2-devel-en] segfault from rbgtk-tree-view.c

Reported by Detlef Reichl. Thanks!!!
goocanvas: replace sample scalability-demo by GI
Merge pull request #157 from myokoym/goocanvas-replace-sample-scalability-demo-by-gi

goocanvas: replace sample scalability-demo by GI

Patch by Masafumi Yokoyama. Thanks!!!
gstreamer: support Gst::Bus#set_handler

But it causes SEGV... We should not use GVL lock/unlock?

[ruby-gnome2-devel-ja]

Reported by Takashi Nakajima. Thanks!!!
gstreamer: extract common code
gstreamer: sample streams