share initial gee assets on AssetSelect widget

[dfguerrerom]

While testing AssetSelect performance, I noticed that it was taking way too long to load the component, and it is because of two reasons:

• the component requests all the user assets by calling a recursive function, this is good, but when the user has several folders, the process takes more time.

• the component observes the types traitlet with the decorator, this causes a double call of the _get_items() function:

  • the first happens at the initialization stage with _get_items and the second happens when the type trait changes from empty to the default, that can easily be fixed if we observe at the init function instead of using the decorator.

• finally, the problem is even worst if we create apps with more than one AssetSelect component, since it will do the same operation several times.

this is also related to: #455

[dfguerrerom]

closed by #932