Move server calls to MusicServerClient and give players stable ids

Server requests move out of Settings into a new MusicServerClient
autoload with one function per endpoint. It sends requests through a
swappable HttpTransport so tests can fake the server, and it frees its
HTTPRequest nodes after use.

Players now have a UUID that never changes. Their position in
Settings.player_array is only used for the Alt+<number> keybinds.
Previously the position doubled as the id, so removing a player made
points and wins land on the wrong player, and could delete another
player's point from a song.

Also fixes:
- format_time showed 59.6 seconds as "00:60"
- searches containing regex characters such as "Q*bert" broke search
- SongObject and PlayerObject were Nodes that were never freed; they
  are now RefCounted
- search matching is split out of the UI code into find_matches()

http_transport.gd

@@ -0,0 +1,37 @@
+class_name HttpTransport
+extends RefCounted
+## Sends HTTP requests for [code]MusicServerClient[/code] using [HTTPRequest] nodes.
+##
+## Tests subclass this to answer requests without a network connection.
+
+var _owner: Node
+
+
+func _init(owner: Node) -> void:
+    _owner = owner
+
+
+## Requests [param url] and calls [param on_complete] with
+## [code](response_code: int, body: PackedByteArray)[/code]. Connection errors are
+## reported with response code 0.
+func request(url: String, method: int, on_complete: Callable) -> void:
+    var http_request: HTTPRequest = HTTPRequest.new()
+    _owner.add_child(http_request)
+    http_request.request_completed.connect(
+        func(
+                result: int,
+                response_code: int,
+                _headers: PackedStringArray,
+                body: PackedByteArray,
+        ) -> void:
+            http_request.queue_free()
+            on_complete.call(response_code if result == HTTPRequest.RESULT_SUCCESS else 0, body)
+    )
+    var headers: PackedStringArray = []
+    if method == HTTPClient.METHOD_PUT:
+        headers.append("Content-Type: application/json")
+    var error: int = http_request.request(url, headers, method)
+    if error != OK:
+        http_request.queue_free()
+        push_error("Could not send request to %s (error %d)" % [url, error])
+        on_complete.call(0, PackedByteArray())