Add "Delete Tag" menu option to tags in the Tag Database and tag box

[Darxoon]

I added this menu option to the tag database and tag boxes. It will prompt the user and if they say yes, delete the tags and remove all references to it

One thing I added is that when you hold shift while clicking Delete Tag, the prompt and popup after it was deleted will be skipped. I figured this would be useful when deleting a lot of tags at the same time.

[Archina]

Would it be possible to add something like a reactive store to handle updates of the tags and similar structures instead of pushing signals back and forth.

This would relax the coupling between the TagDatabase and the TagWidget. It would also make it easier to listen to such changes in other places, like under the image preview in the main tag browser.

However, I am more of a Frontend Dev than a python dev.

[Darxoon]

I implemented your suggestions that system tags can't be deleted now