videodb: fix issues with tvshowview by integrating "GROUP BY tvshow.idShow" into the CREATE VIEW statement

This fixes a few cases missed by #4977. The most prominent bug introduces was that counting the total number of tvshows didn't work anymore (see http://forum.xbmc.org/showthread.php?tid=68263&pid=1770810#pid1770810).

I'm not sure why the GROUP by tvshow.idShow wasn't added to the CREATE VIEW for tvshowview in the first place. I've looked over all places where tvshowview is used and couldn't find one that wouldn't be fine with this change.