Don't presume that notify events are sent only on change

The piece of code that keeps track of the number of "interesting windows" uses a simple counter to do it. But for this to work it is mandatory that the notify::skip-taskbar signal is sent only if the value has really changed. If it is emited but the value hasn't been modified, it will fail and calculate an incorrect value. This means that a piece of Javascript code that overwrites the skip-taskbar property with new code won't be able to send change notifications, because the change would be visible only to the dynamic code, but not to the C code.

This patch fixes it.

Fixes #2224