Add HScrollbar to ItemList when max_columns == 0

[havi05]

I implemented this proposal godotengine/godot-proposals#6558 to create an unlimited number of columns if the variable max_columns is set to 0 and to add a horizontal scrollbar if the size of the node is too small.

This PR doesn't solve #56045 so the items are still selectable at the margin and the text is not cut off at the left and bottom margins. The workaround mentioned in this comment #56045 (comment) still works.

Fixes godotengine/godot-proposals#6558

[havi05]

I have added an wraparound_items boolean, because I noticed that the horizontal scroll bar was breaking some UI's, like the file selection dialog of the Project Manager. (All Items were in a row before the last force-push and did not fill the space.)

The boolean wraparound_items is set to true by default and dosn't change the behavior of the ItemLists (compared to before this pr), so it shouldn't be a breaking change.

If it is set to true, all items will be moved to the next row, if there isn't enough space. (as was the case before this pr.)
If it is set to false, a horizontal scroll bar is added and the maximum scroll value is set to show all columns set in max_column.
(If wraparound_items = false and max_columns = 0 unlimited columns will be generated.)

Please let me know if you have any suggestions for improving the wording of the documentation.

[akien-mga]

Thanks!