[Feature Request] Real tree support

[andreyorst]

Currently kaktree only imitates the tree, by doing requests to the filesystem only when user expands the directory, and replacing the contents of the buffer with Kakoune. This means that we currently are not able to refresh the whole tree, and if we fold something that has unfolded directories inside it, on unfolding it again those directories will not be unfolded.

This FR proposes to create and maintain real filetree in some kind of variable, and make all requests based on the information of that tree. This way folding and unfolding can still be done by Kakoune replacement commands, but correct trees will be obtained. This will also allow refreshing full tree from top to bottom.

Work on this feature will be going in real-tree branch.