Unloading of unused web resources

[ksons]

Resources are cached in an optimized data structure because they can be used from multiple places inside the scene. The resource manager should track all users of a resource and free the cached resource as soon as no user exists for the resource anymore.

[ksons]

This behavior is the intended behavior of XML3D, thus it is a bug if resources are not unloaded automatically.

Due to many locations where we cache resources for performance reasons and the lack of weakmaps until recently, the garbage collector is not able to free all resources.

TODO: Check all locations where resources are cached and replace objets with WeakMap where appropoate

[csvurt]

Moved this to the resource manager overhaul scheduled for 5.1

[csvurt]

We identified a lot of areas where stray references were preventing garbage collection of resources after elements were removed from the DOM. This was improved quite a bit in 5.1, memory profiling in Chrome is now relatively stable even when adding and removing hundreds of elements per second.