Synview: Preload resources

[chird]

Improvement within the preload options in the Type class can be made.

• The preload() method is up to now only implemented in the Image class.
• Preload strategy is currently fixed, all resources are immediately loaded.

Preload options should be expanded with the following keys:

strategy: 'all' | 'nearest', // 'nearest' has to be implemented in the Synview class.
maxPendingResources: number | Infinity, // how many resources are loaded at once
maxPreloadedResources: number | Infinity // for 'nearest' -> how many resources are preloaded "around" the current visible timestamp.