Description:
Tags are great feature of Piwigo but one feture it make more better. It is negative tags. FE I want find Natural pictures but without Buildings or Natural without People. It isnt possible now. I have idea - Negative tags

Example:
For example in demo - I want Celebrations without Military but it is impossible - http://piwigo.org/demo/index.php?/tags/58-Celebrations

Functionality suggestion:
One click - checked (positive select)
Second click - checked by cross (negative)
Next click - unchecked

Graphic design suggestion:


thank you for consideration