Interface to add places to a category
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In the edit category form now you can add places with latitude and longitude to a database. As many places as desired. It saves nicely to the data base.

Known issues:
[ ] Can't remove a place.
[ ] Problems when using non ascii characters.
Conditional loading

Add filters and actions only if the plugin is active.
include admin.php always
singleton
First draft of category widget
It still lacks functionality but the basic thing is there: given a list of visited places, they are shown in a map.
Merge branch 'testing'