move marker to separate module

src/marker.mli

@@ -0,0 +1,38 @@
+type t = [ `Marker ] Layer.t
+
+(** type for marker option, used to create marker *)
+type marker_opt =
+  | Icon of Icon.t
+  | Keyboard of bool
+  | Title of string
+  | Alt of string
+  | Z_index_offset of int
+  | Opacity of float
+  | Rise_on_hover of bool
+  | Rise_offset of int
+  | Pane of string
+  | Shadow_pane of string
+  | Bubbling_mouse_events of bool
+  | Auto_pan_on_focus of bool
+
+(** [create_marker latlng options] is a new marker with the same position as
+    [latlng] and with options set to [options] *)
+val create_marker : Latlng.t -> marker_opt list -> t
+
+(** Returns the current geographical position of the marker. *)
+val get_latlng : t -> Latlng.t
+
+(** Changes the marker position to the given point. *)
+val set_latlng : Latlng.t -> t -> unit
+
+(** Changes the zIndex offset of the marker. *)
+val set_z_index_offset : int -> t -> unit
+
+(** Returns the current icon used by the marker *)
+val get_icon : t -> Icon.t
+
+(** Changes the marker icon. *)
+val set_icon : Icon.t -> t -> unit
+
+(** Changes the opacity of the marker. *)
+val set_opacity : int -> t -> unit