Enable scroll hints for several parts of the editor

[YeldhamDev]

Tackles https://github.com/godotengine/godot-proposals/issues/13581 and #112454.

• Enable scroll hints for several areas in the editor where it made sense and looked appropriate.
• Only present in the modern theme.
• Add scroll hints for ItemList, necessary for this PR to be made.
• Fix some issues with scroll hints in Tree.
• Remove the scrollbar hack in the animation player editor, putting it back in its default place. Instead, it adds a gap in the timeline control when the vertical scrollbar is visible while in RTL mode.

Screenshots

[KoBeWi]

The relationship lines in FileSystem don't reach the bottom:

EDIT: Also it would be nice to add hints to CreateDialog too. It seems to be missing in the changes.

EDIT: Also ResourcePreloader xd

[YeldhamDev]

@KoBeWi I can't really add it to the create dialog because the hints wont be able to touch the left border due to the lists in the way.

Also, is there a way for an EditorDock to know if it was placed in a dock or in the bottom panel? I need this information to properly set the hints for the ResourceLoader.

The relationship lines in FileSystem don't reach the bottom

This doesn't seem related to the PR, but instead a bug in Tree.

[KoBeWi]

Also, is there a way for an EditorDock to know if it was placed in a dock or in the bottom panel?

Override update_layout(). DOCK_LAYOUT_HORIZONTAL is for bottom panel.

[YeldhamDev]

@KoBeWi Changes made.

[akien-mga]

Thanks!