[ios] Make user heading indicator property IBInspectable

Rename it to `showsHeading` to fit in the tiny label column of the utilities panel.

platform/ios/src/MGLMapView+IBAdditions.h

@@ -41,6 +41,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic) IBInspectable BOOL allowsRotating;
 @property (nonatomic) IBInspectable BOOL allowsTilting;
 @property (nonatomic) IBInspectable BOOL showsUserLocation;
+@property (nonatomic) IBInspectable BOOL showsHeading;
 
 @end
 

platform/ios/src/MGLMapView.mm

@@ -5775,4 +5775,19 @@ private:
     self.pitchEnabled = allowsTilting;
 }
 
++ (NS_SET_OF(NSString *) *)keyPathsForValuesAffectingShowsHeading
+{
+    return [NSSet setWithObject:@"showsUserHeadingIndicator"];
+}
+
+- (BOOL)showsHeading
+{
+    return self.showsUserHeadingIndicator;
+}
+
+- (void)setShowsHeading:(BOOL)showsHeading
+{
+    self.showsUserHeadingIndicator = showsHeading;
+}
+
 @end