Fix JS types in more callbacks

Fixes the following JSUnit tests on PPC:

test0040mainloop.js
testByteArray.js
testCairo.js
testEverythingBasic.js
testFundamental.js
testGDBus.js
testGObjectClass.js
testMainloop.js
testParamSpec.js
testTweener.js

Signed-off-by: Michel Dänzer <michel@daenzer.net>

https://bugzilla.gnome.org/show_bug.cgi?id=729554
importer: Add back the ability for others to install overrides

Some third-party libraries rely on the ability to install the overrides,
so let's let them.