Fix frame dragging being blocked when it overlaps another frame

[cccraig]

If two faces are close together one face can be successfully tagged but the second face may be blocked because the first tag frame has a higher z-index than the image. This blocks the mouse over event and halts the dragging of the second frame. This can cause other issues, like releasing the mouse button but the frame drags with you.

Proposed fix is to dynamically change the z-index of previously created frames to hide them while the new frame is being drawn. Then revert the z-index after name has been entered.

[cccraig]

Fixed this with some CSS magic and the :not selector. This has resulted in the manifestation of some other behaviors that may / may not need fixing. As an example, suppose you take a group photo with the "Work Squad" and you tag all the people in it with their name. Later, for whatever reason, you decide to select the entire group and tag it as Work Squad. Doing this will work and retain the old tags but hovering over the tagged region would only show "work squad". This specific behavior should be fixed by displaying all tag names behind the selector area.

Secondly, you can't start tagging by clicking inside another tag. The overlap area is okay, if you click outside of another frame you can then drag into it and won't be blocked. I'm inclined to leave this behavior as is; if you've got another tag within a tag you've really not using the plugin as intended.