Add cell/button visibility control to TreeItem

[DexterFstone]

Description:

This PR adds cell/button visibility control to the TreeItem, allowing for hiding or showing individual cell contents without affecting the overall tree layout. This PR will help implement something like #10911 in the future.

Methods:

TreeItem.set_cell_visible(column: int, visible: bool) -> void:
TreeItem.is_cell_visible(column: int) -> bool:
TreeItem.set_button_visible(column: int, button_index: int, visible: bool) -> void:
TreeItem.is_button_visible(column: int, button_index: int) -> bool:

Signal:

Tree.item_hovered(item: TreeItem, column: int)

The signal is the main part of implementing the cell visibility change, for example, hovering over a cell shows the button, etc.

[KoBeWi]

How does it help the mentioned proposal if all buttons are inside a single cell?

[DexterFstone]

How does it help the mentioned proposal if all buttons are inside a single cell?

I updated the code; now it is possible to hide the specific index of the button using set_button_visible.