Add GridMap item categories

[smix8]

Adds GridMap item categories and category tree view.

How does it work?

MeshLibrary has a new item property category and related setgets set_item_category() and get_item_category().

This string is used to create the new category tree view.

If the value is left empty (default) the item will be added to the default "All" tree root item.

A category string can include "/" as a delimiter. This delimiter splits the string and turns the splits into tree sub categories.

For a quick overview how many items exist in a category the number of the available items in each category is shown to the right of the item tree view in the secondary column.

The search box was updated to filter for both item names and item categories.

MeshLibrary scene exporter

The MeshLibrary scene exporter now reads the node metadata of each exported MeshInstance3D node.

If it finds a category key with String(Name) value this string will be added as the category for the exported item.