JNI: implement DetachCurrentThread function
JNI: implement DetachCurrentThread function

This method still needs to be appropriately called in lots of
places to be of use, but in principle the attached threads ought
to be detached so here it is.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=745312

thisMagpie/java-atk-wrapper,GNOME/java-atk-wrapper,GNOME/java-atk-wrapper,GNOME/java-atk-wrapper,thisMagpie/java-atk-wrapper,thisMagpie/java-atk-wrapper