glib2: don't call g_thread_init() when is uses glib 2.32.0 or higher

glib thread system is automatically initialized since glib 2.32.0.

ref: https://developer.gnome.org/glib/2.42/glib-Deprecated-Thread-APIs.html#g-thread-init
gi: support GList of UTF-8

[ruby-gnome2-devel-en] gtk3-gi crashes on Gtk::IconTheme#list_icons

Reported by Detlef Reichl. Thanks!!!
glib2: add missing parenthesis
glib2: support format_size

default unit system is KILOBYTE_FACTOR(1000).
gtk3 test: suppress initialize style warning

```log
in `new': 'Gtk::Image#initialize(image, size = nil)' style has been deprecated.
Use 'Gtk::Image#initialize(:label => nil, :mnemonic => nil, :stock => nil, :size => nil)' style.
```
glib2: remove trailing spaces
glib2: support flag argument in GLib.format_size
glib2: remove trailing spaces
glib2: change GLib.format_size API style

secodary argument expects Hash instead of Enum:

GLib.format_size(1024 * 1024 * 1024, GLib::FormatSizeFlags::IEC_UNITS) ->
GLib.format_size(1024 * 1024 * 1024, :flags => :iec_units)
glib2; Indent
glib2: don't combine explicit brace style and omitted brace style
glib2: use meaningful variable name
glib2: use "rb_" prefix for Ruby value variable name

Because other variable uses it in the function.
glib2: remove needless check

rbg_scan_options() does it.
glib2 test: group format_size tests
glib2 test: indent
glib2 test: check GLib version in setup
glib2 test: split tests

One test should check only one thing.
glib2 test: remove needless tests
glib2 test: group :flags => :iec_units tests