Allow adding a custom side menu to EditorFileDialog

[aaronfranke]

This PR adds a method to EditorFileDialog to add a custom side menu. You can specify a title text, which will be themed similarly to the rest of EditorFileDialog, and a Control node to be added below it.

I am planning to use this functionality to add export settings for GLTF. Here is a screenshot of this in action:

[aaronfranke]

Updated the PR based on feedback, the side panel will now go all the way to the bottom, with the file type dropdown always under the list of files.

[YuriSizov]

I haven't tested it, but code looks alright.

We may want to add a method to remove the sidebar as well, since it won't be enough to just remove the control from its parent. You need to remove the title as well, which is created implicitly. It's not impossible to work around, but hooking into the exact node tree structure would be less than ideal when we could just expose a clear_side_menu method that would handle everything or us.

[aaronfranke]

Appearance with the HSplitContainer:

[akien-mga]

Thanks!