Fix toolview hiding/showing

I can't even begin to describe how broken this was:

• emptyChanged() was not emitted when a button was added
• there was a timer firing every 100ms, always (!) checking whether to show the toolview
• once a toolview was hidden the member storing the state could never change such that it would show again

CCMAIL:kfunk@kde.org

This _probably_ fixes
BUG:364590
because I suspect it happens when the toolviews are constructed before
plugins are loaded; then there are no buttons, and they are all hidden,
and will never be shown again.