Description of problem:

If you haven't selected any packages to add or remove, the "Apply" button should
be inactive.

I have seen lot of users that, after installing software press "Apply" one extra
time, in the belief that there was more to do. The result is that they get a new
dialog box telling them that there is no changes to apply and an extra "OK"
button to click on.

This is poor usability, if the button hadn't been active there would have been
no need for the dialog, and the user would have saved two mouse clicks and the
trouble of reading the pop up dialog text.