gio2: add Gio::Dbus module

This module contains Gio DBus Utilities methods such as:
* ganerate_guid
* is_*

ref: https://developer.gnome.org/gio/2.42/gio-D-Bus-Utilities.html

(Gio::DBusUtilty or Gio::DBusUtilties is more appropriate for this
module name?)
gio2: use rubyish_method_name
gi: don't treat have argument get_XXX method as predicate method
gio2: use rubyish_method_name with :prefix
glib2 windows: rename to "windows" from "win32"

We'll support 64bit Windows.
glib2 windows: use "Windows" for class name
windows: use "windows" instead of "win32"

It's not tested. Sorry.
windows: make RUBY_CC_VERSION customizable
glib2: fix invalid alias_method usage

Object doesn't have alias_method.
glib2: fix a typo
glib2: really fix a typo
glib2 native: set PKG_CONFIG_PATH in "make"

Because GObject Introspection searches GLib and so on by pkg-config.
gi native: avoid double patched
gi native: avoid double patched
gtk2 windows: remove PRIVATE symbols from .def

Because MinGW-w64 reports errors for nonexistent symbols.
gstreamer windows: follow name change
glib2 windows: remove PKG_CONFIG_FOR_BUILD

Because GTK+ 3 seems that building executables on build environment
doesn't work.
gstreamer windows: follow name change
gstreamer windows: fix a typo...
Add old style set_rgba with separate values