goocanvas sample: use more specific name

Fix undefined method `prepend_environment_path'

ref: commit 510a0c216688d0209ecd5c8ec33e8eeaab3a07ee
Merge pull request #152 from myokoym/use-more-specific-name

goocanvas sample: use more specific name

Patch by Masafumi Yokoyama. Thanks!!!
gi: use "value" instead of "in"

"in" is too generic. There are other input argument type such as
function argument and callback argument.
gi: use "in" instead of "call"

It should support all input argument conversion.
glib2: support unowning boxed object
gi: start supporting onwership tranfer for input argument

Struct and Object are only supported for now.
goocanvas: replace sample table-demo by GI
glib2: add a macro for suppress a warning for "%p"

It is used like:

    rb_raise(rb_eArgError,
             "must specify one of them: "
             "[:stock, :icon_name, :icon_set, :gicon, :file, :pixbuf, "
             ":animation]: %p",
             RBG_PRINTF_INSPECT_VALUE(rb_options));
gtk3: make icon size optional for Gtk::Image#set_*
Merge pull request #153 from myokoym/goocanvas-replace-sample-table-demo-by-gi

goocanvas: replace sample table-demo by GI

Patch by Masafumi Yokoyama. Thanks!!!
gi: make load_field customizable easily
gi: indent
goocanvas-gi: add child reference to parent to guard from GC
goocanvas-gi: remove needless GC control
gi: add missing "i" argument

GitHub: fixes #154

Reported by Masafumi Yokoyama. Thanks!!!
goocanvas-gi: follow gi change
goocanvas: replace sample widgets-demo by GI
Merge pull request #155 from myokoym/goocanvas-replace-sample-widgets-demo-by-gi

goocanvas: replace sample widgets-demo by GI

Patch by Masafumi Yokoyama. Thanks!!!
goocanvas-gi: remove_child accepts child widget
gstreamer: sample stepping