fix: eagerly listen to `TileLayer.reset` stream to ensure handler is fired

[JaffaKetchup]

Also added refreshing of reset listener when reset stream changes within widget lifecycle.

Fixes #1808. However, the behaviour is still not correct, because the tile loading logic is incorrect. This looks like #1813:

• If the urlTemplate does not change, tiles are evicted but not loaded until the map is moved (and possibly the tiles are not evicted correctly because they appear to reload quickly, so maybe from cache, but reset should empty cache)
• If the urlTemplate does change, tiles are evicted but then instantly reloaded (see above), and new tiles load in gradually replacing/overlaying the old ones - the old tiles should not become visible in-between

Therefore, this will need a follow up PR.

[JaffaKetchup]

We can add a more detailed test for the pruning etc. logic when #1813 is fixed.