Hidden export hint for variables used only in PackedScenes

[mrzapp]

I'd like to propose a way for exported variables not to show up in the inspector. This is for when the variable needs to be saved into a PackedScene, but should not be editable by designers.

An example of such a variable could be the state of an NPC.

In order to not mess with the parser too much, it could be added as a keyword, like this:

export hidden (STATE) var state : int = STATE.IDLE

[bojidar-bg]

Duplicate of #5988. (even the export hidden idea: #5988 (comment). Great minds...)