gtk2 rbgdkdragcontext.c: drop GTK_CHECK_VERSION(2,6,0)
gtk2 rbgdkwindow.c: drop GTK_CHECK_VERSION(2,6,0)
gtk2 rbgdkevent.c: drop GTK_CHECK_VERSION(2,6,0)
gtk2 rbgdkdraw.c: drop GTK_CHECK_VERSION(2,6,0)
gtk2 rbgdkpangorenderer.c: drop GTK_CHECK_VERSION(2,6,0)
Merge pull request #188 from cosmo0920/drop-gtk2.6.0-macro-gdk-part

drop GTK_CHECK_VERSION(2,6,0) macro in gtk2/rbgdk*.c

Patch by cosmo. Thanks!!!
gtk2: untabify
gtk2: remove a trailing space
gtk2: support running gtk-demo sample from other directory
gtk2: fix indent
gtk3: untabify
gtk3: remove a trailing space
gtk3: remove wrong information from an error message
gtk3: fix indent
gtk2: implement GdkEvent class hierarchy by defining sub-types

GitHub: #180

TODO:
  * Fix styles
  * Backport to Ruby/GDK3
gtk2: use predefined classes
gtk2: expand an abbreviated variable name
gtk2: use "!XXX" instead of "XXX == NULL" for NULL check
gtk2: don't use one-line "if return"

It is not gdb friendly. We jump to the end of function by "next"
command when the "if" condition is true. If the "if" condition is
unexpected, we want to confirm the information in the local block
scope. But if we jump to the end of function, we can't confirm the
information. Because we are already in outside of the local block.
gtk2: remove trailing spaces