[lively-kernel] Part caching

[Lauritz Thamsen]

Hi.

We introduced simple caching of essential parts as, for example, the script editor and the explorer. Cache hits are possible on loading parts programmatically as, for example, the world's menu and some halo buttons do, while the parts bin browser always gets you the latest version.

Also, the world's menu offers controls to disable caching temporarily and to clear the cache, see "Preferences". 

Please let us know, if this change introduces any inconveniences for you.

Best,
Lauritz