Unify PlatformTarget.watch().

The implementation was copy-pasted, so it makes more sense to have only a single copy. `HTML5Platform` added a comment about possible future changes, so I kept that. Future changes will still be possible by overriding the function.

tools/platforms/TizenPlatform.hx

@@ -261,4 +261,6 @@ class TizenPlatform extends PlatformTarget
 	@ignore public override function install():Void {}
 
 	@ignore public override function uninstall():Void {}
+
+	@ignore public override function watch():Void {}
 }