Add create map and copy map functions to weathermap-cacti-plugin-mgmt.php

[nbz6]

This is an attempt to fix #144

Probably not perfect but should fix the missing create map and copy map function without too many changes. I've included some part of the show_editor_startpage() functions from lib/editor.inc.php and lib/editor.actions.php, these functions may be later deleted to clean up old unused code if the direct access to weathermap-cacti-plugin-editor.php is deprecated in this only Cacti's plugin.

This also makes it clearer for users who want to add a new weathermap, the "+" button opened only a management of .conf file but did not allow in this same interface to add a new one from scratch, they had to go in another interface in the past. The duplicate function also don't allow to rename .conf so to keep a clear naming of all .conf it is not ideal, the copy function is better for that too.