don't leak so much memory when changing between feeds.

totally unscientific measurements done with ksysguard and by swithing between all feeds (9189 total/35011 unread) and slashdot (3435/3930):
startup: 35,048K, switch to all: 44,336K, slashdot: 41,876K, all: 44,824K, slashdot: 42,696K

does still seem to leak, but nothing like it used to be (switching between all and slashdot it took approx 8MB)