Don’t require `bundle exec' for the executables by requiring bundler first.

So you can now do
  $ bin/openrubyrmk-gtk
directly rather than
  $ bundle exec bin/opengrubyrmk-gtk
Exit mdump with error on error
To better resemble the layer layout, order the layer list reversely.

That is, upper layer are shown at the top of the list, lower ones
at the bottom.
Update backend library.
Fix issues related to updating to the official ruby-tmx.
Merge branch 'master' of github.com:OpenRubyRMK/gtk-frontend