Remember state of Browser/AddCards window if already open

Hello,

The source of this submission is the following topic: https://anki.tenderapp.com/discussions/ankidesktop/8091-search-duplicates-always-open-a-new-browser

In fact, Anki does not open a "new" browser as I thought it did. Instead it sets the WindowState (which is a combination of flags: http://qt-project.org/doc/qt-4.8/qt.html#WindowState-enum) to the WindowActive flag, thereby resetting all the other flags to 0. If the window was maximized (as one may frequently want to for the browser?) it instead gets to a non-maximised state. The present commit corrects this behaviour.

Any question or comment, please let me know.