Ability to set entities/keyvalues as hidden in the FGD

[vrad-exe]

Which component should be improved?

Hammer

Describe your feature suggestion in more detail

Add a hidden tag that can be specified somewhere in the FGD for an entity or keyvalue to make it not show up by default. This would mainly be used for deprecated features that need to be maintained for backward compatibility but shouldn't be cluttering up the UI.

For entities, this would prevent the entity from appearing in the various dropdown lists of entity types; the user would need to manually type in the name if they wanted to place one. However, any copies of the entity still in the map would continue to show as a valid entity. The hidden flag would be set as a helper, without parentheses (like halfgridsnap).

For keyvalues, they would be hidden if set to their default value. If the keyvalue was set to a non-default value, it would appear in the list with a yellow background. Note that the keyvalue would still be saved into the VMF/BSP, it would just be hidden from Hammer's UI. Hidden keyvalues should also still appear when SmartEdit is turned off. The hidden flag would be set in the same place as readonly.

For example:

@PointClass base(BaseEntityPoint) hidden = "hidden_entity" : "Example hidden entity"
[
	hidden_keyvalue(integer) hidden : "Example hidden keyvalue" : 0
]

[tmob03]

This would go along with the issue I have open in the fgd repo, for a small scripting Lang in the fgd, and this can be a feature