Improve API to add new persistent template pathes

[onsetsu]

Current custom template pathes can be added using either:

lively.components.addTemplatePath(lively4url + "/your/path/")

• good enough API
• path not persisted through pages-reloads

OR

localStorage.lively4customTemplatePaths = JSON.stringify(currentPaths);

• no dedicated API
• path persisted through pages-reloads

For persisted paths, add a usable API to ComponentLoader:

• [x] wrap localStorage access
  • [x] automatically convert JSON from/to String
• [x] on new paths, bust template caches using lively.components.resetTemplatePathCache()
• [x] methods for adding, removing, gettingAll persisted custom paths
• [x] when reading template paths, allow for paths starting with / and prefix them with lively4url