programmable buttons in the media viewer to apply/remove tags

[PunishedGrimace]

Add buttons to the media viewer that apply/remove tags on a file, similar to how there's buttons for the rating system. Similar to #513 but with buttons in the media viewer. This would make processing a large number of images and dealing with a handful of commonly used tags much faster.

• Each button is for a single tag (or multiple tags, but I have no idea how that would work)
• The button is large enough to display the tag inside it as text
• The button is lit if the file already has that tag or one of its siblings
• Clicking the button while it's unlit applies the tag, clicking the button while it is lit removes the tag

Attached is an image to illustrate what I'm thinking of. The new buttons are in the top right, near the rating buttons and the archive button.

[bbappserver]

Related: https://github.com/hydrusnetwork/hydrus/issues/273 (Apply Preset Tags To Selection)