added parent field in edit category form (#14)

sub categories not implemented sucessfully
added selection option for no parent category ("None") in edit category form

added parent field in cat_data for top level categories also
added 'level' field in cat_array; added indenting of sub categories in cat table
added sorting of cat_array (not fully implemented yet)
fixed saving categories
added sort function for categories; show correct order in category table
added subcategory support for parent combobox in edit category form
added sub category support in new event category selection
small changes in get_category_string function

issue #14 should be finished with this commit
moved category administration to seperate page; also splitted affected classes
fixed path to category admin page
small improvements in help texts on about page
preparation for version 0.4.4
Merge branch 'develop'