Hardcode prefix instead of using constant.

This makes the code look cleaner and also makes it easier for developers to search the code for 'bgmp_filter_example', etc.


git-svn-id: https://plugins.svn.wordpress.org/basic-google-maps-placemarks/trunk@884814 b8457f37-d9ea-0310-8a92-e5e31aec5664