Seperate signs API from signs definitions

Change modnames from *_lib to *_api

display_api/API.md

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+# Display Lib API
+This document describes Display Lib API. Display Lib allows to add a dynamic display on a node. Display Lib limits node rotations. For wallmounted, only vertical positionning is available, and for facedir, only first four position are availabel (those with default axis).
+
+## Provided methods
+### update\_entities
+**display\_lib.update\_entities(pos)**
+
+This method triggers entities update for the display node at pos. Actual entity update is made by `on_display_update` callback associated to the entity.
+
+`pos`: Position of the node
+### register\_display\_entity
+**display\_lib.register\_display\_entity(entity_name)**
+
+This is a helper to register entities used for display. 
+
+`entity_name`: Name of the entity to register.
+## Provided callback implementations
+### on_place
+**display\_lib.on\_place(itemstack, placer, pointed\_thing)**
+
+`on_place` node callback implementation. Display nodes should have this callback (avoid placement of horizontal display node).
+### on_construct
+**display\_lib.on\_construct(pos)**
+
+`on_construct` node callback implementation. Display nodes should have this callback (creates, places and updates display entities on node construction).
+### on_destruct
+**display\_lib.on_destruct(pos)**
+
+`on_destruct` node callback implementation. Display nodes should have this callback (removes display entities on node destruction). 
+### on_rotate
+**display\_lib.on\_rotate(pos, node, user, mode, new_param2)**
+
+`on_rotate` node callback implementation. Display nodes should have this callback (restricts rotations and rotates display entities associated with node).
+### on_activate
+**display\_lib.on_activate(entity, staticdata)**
+
+`On_activate` entity callback implementation for display entities. No need of this method if display entities have been registered using `register_display_entity` (callback is already set). 
+
+## Howto register a display node
+* Register display entities with `register_display_entity`
+
+* Register node with :
+  - `on_place`, `on_construct`, `on_destruct` and `on_rotate` callbacks using display_api callbacks.
+  
+  - `display_modpack_node` group. This will make this node have their entities updated as soon as the mapblock is loaded (Useful after /clearobjects).
+  
+  - a `display_entities` field in node definition containing a entity name indexed table. See below for description of each display_entities fields.
+
+### Display_entities fields
+`on_display_update` is a callback in charge of setting up entity texture. If not set, entity will have no texture and will be displayed as unknown item.
+
+`depth`, `right` and `heigh` : Entity position regarding to node facedir/wallmounted main axis. Values for these fields can be any number between -0.5 and 0.5 (default value is 0). Position 0,0,0 is the center of the node. `depth` goes from front (-0.5) to rear (0.5), `height` goes from bottom (-0.5) to top (0.5) and `right` goes from left (-0.5) to right (0.5).
+
+In order to avoid flickering text, it's better to have text a little behind node surface. A good spacing value is given by `display_api.entity_spacing` variable.
+
+### Example
+
+	display_api.register_display_entity("mymod:entity1")
+	display_api.register_display_entity("mymod:entity2")
+
+	function my_display_update1(pos, objref) 
+		objref:set_properties({ textures= {"mytexture1.png"},
+		                        visual_size = {x=1, y=1} })
+	end
+
+	function my_display_update2(pos, objref) 
+		objref:set_properties({ textures= {"mytexture2.png"},
+		                        visual_size = {x=1, y=1} })
+	end
+
+	minetest.register_node("mymod:test_display_node", {
+		...
+		paramtype2 = "facedir",
+		...
+		groups = { display_modpack_node = 1, ... },
+		...
+		display_entities = {
+			["mymod:entity1"] = { 
+				depth = 0.3,
+				on_display_update = my_display_update1 },
+			["mymod:entity1"] = { 
+				depth = 0.2, height = 0.1,
+				on_display_update = my_display_update2 },
+			},
+		...
+		on_place = display_api.on_place,
+		on_construct = display_api.on_construct,
+		on_destruct = display_api.on_destruct,
+		on_rotate = display_api.on_rotate,
+		...
+	})