Fix TabbarWidget buttons brokennes

Commited on behalf of blaze (Mikołaj Płomieński)

REVIEW: D5759

Fixes a problem in TorrentActivity panel, when the button is checked (pressed) at the startup but the panel is "shrunken" (not visible). Button's state should correspond to the panel visibility.

Steps to reproduce:

Run KTorrent
Open any of the Torrent Activity panels at the bottom of the window
Hide it
Quit KTorrent
Run KTorrent again
Torrent Activity previously selected is checked (pressed) but no open panel. In order to open this panel you have to click the button two times