/explore: search results rebuild during load optimization

[dognotdog]

the search results list being rebuilt while all feeds are loaded causes some annoying temporary UI glitches.

Adding a further optimization that doesn't rebuild the list unless the items already displayed have changed seems reasonable, as that should prevent the arrival of additional feeds that would not be visible anyway from interfering with what's on screen.

Doing that should also fix #1 as the theory is that that happens when the list is being rebuilt before the click on the checkbox is processed, as the rebuild can take a significant fraction of a second during which UI events are blocked.