Simplify FeedView._fillWindow(), part 1.

lastEntryInCenter check can be factored in enoughEntriesPreloaded.
This not only simplifies the code but also makes FeedView._onEntriesAdded()
take lastEntryInCenter into account which it should have done.
Simplify FeedView._fillWindow(), part 2

The early return is unnecessary, it's simpler to just invert the conditions.
Fix FeedView._fillWindow() to respect aWindowHeights argument.

The constants WINDOW_HEIGHTS_LOAD and INITIAL_WINDOW_HEIGHTS_LOAD
didn't actually work because of this.
FeedView._onEntriesAdded() should reset _allEntriesLoaded.