[plugin] Save tree nodes to cache and fetch the nodes if they were changed

[vinokurig]

Description

Currently the tree plugin API always requests tree nodes from the plugin side. We need to store the tree nodes in a cache once they were fetched and return only cached nodes. Cache must be updated by the plugin event. This will increase the tree performance.

Reproduction Steps

1. clone and compile https://github.com/microsoft/vscode-extension-samples/tree/master/tree-view-sample
2. Start the hosted plugin, open the tree view.
3. Expand and collapse the same tree node.

Children request is sent on each expand : https://github.com/theia-ide/theia/blob/7406add2a60106eaeba41a9b117d16e90c380764/packages/plugin-ext/src/plugin/tree/tree-views.ts#L91
OS and Theia version:

Diagnostics:

[akosyakov]

@vinokurig Do you experience perf issues somewhere to introduce optimization complexity? It would be good to justify it first, i.e. with reproducible example.

[vinokurig]

It is actual for vscode github-pull-request plugin. The plugin sends requests to GitHub on each pull-request node click, even if it was opened earlier, while in vscode the nodes are requested from cache.

[vinokurig]

This impacts on the tree commands as well:
In the vscode tree command is registered only when the tree node is fetched so they can refresh the commands map on the refresh tree request. In Theia tree nodes are always requested by the tree, so the commands map is refreshed on the children request to not to overload the commands map. See #5961 (comment)