Add filter & sort to editor file dialog

[pafuent]

Closes https://github.com/godotengine/godot-proposals/issues/2721

On EditorFileDialog:

• Add filter box that only shows folders and files in current directory that match
• Add sort button to sort files and directories
• Add a shortcut for CTRL+F for selecting the filter box

Also moved common code between EditorFileDialog and FileSystemDock to it's own file.

This tries to address comments on https://github.com/godotengine/godot/pull/48734 and change it to make it compatible with code on master.