Minor enhancement proposals

[Aranuvir]

• Allow empty tag list. Currently the sex is predefined. When clicking edit tags, without any changes unisex gets selected.
• Add a list of predefined Z-depth values (like in old MC)

[Aranuvir]

I've revised the tag system a bit (branch is aranuvir, the other one got deleted). Now the tag groups available are only controlled by the json file, making the system a bit more flexible and extensible...
This fixes also the first part of my wish list.

Further testing is needed ...