Make use_global_engine adjustable

It has been a long time since [Issue 1568 Provide to disable global engine in ibus 1.5](http://code.google.com/p/ibus/issues/detail?id=1568) was created.

`use_global_engine` hardcoded to `TRUE` and not changable (via dconf) is really annoying to me. And finally I decided to do something despite that I am not familiar with glib / ibus stuff.

This pull request makes `use_global_engine` adjustable via environment variables and DBus interface. No changes to GUI and default values so that average users feel no different. It should be safe to merge these commits.

This is the first step to resolve issue 1568. Next step is to fix ibus-ui-gtk3, which is currently broken with `use_global_engine` set to `FALSE` (ibus-ui-gtk2 works well on this but has other issues such as keyboard shortcuts). And finally, `use_global_engine` option should be brought back to `ibus-setup` interface.