Do not rely on Pythons conversion from int to bool when connecting slots

The stateChanged signal of QCheckBox emits an int but the setEnabled slots
expect a bool.

This is a fix for [PICARD-417](http://tickets.musicbrainz.org/browse/PICARD-417), I've tested it on a Fedora 13 VM with PyQt 4.7.3 and Qt 4.6.3

The other alternative is using old-style signal and slot syntax but well, that's old stuff.

I'm not sure at which version of PyQt or sip the current code starts to work because I didn't find anything useful in the changelogs - sending this as a pull request if someone has any better idea.