Node::get_tree() returns non-null

[Bromeon]

Changes Node API:

fn get_tree(&self) -> Option<Gd<SceneTree>>;

to:

fn get_tree(&self) -> Gd<SceneTree>;
fn get_tree_or_null(&self) -> Option<Gd<SceneTree>>;

Following required object APIs (#1383), we can classify Node::get_tree() in a similar way -- it's supposed to be called inside the tree. Calling it outside the tree is explicitly an error:

SceneTree get_tree() const

Returns the SceneTree that contains this node. If this node is not inside the tree, generates an error and returns null. See also is_inside_tree().

This differentiates it from other methods like get_window, get_parent, get_owner etc. which have explicit non-error branches for the nullptr return type, and thus continue to return Option.

The fallible method is now available under get_tree_or_null. Unlike before, it no longer prints an error in the None case.