v2.1.0 release

• enhancement: add columns to media gallery with option to re-attach/detach and to show/cleanup filesizes (previously separate th23 Media Gallery Extension plugin, but "close" to upload functionality)
• enhancement: option to keep own backup upon upload / as soon as possible, prevent "dormant" copies of images upon usage of built-in image editor, properly use unmarked copy as input for image editor where required, restore original image
• fix: improved css styling of some admin items not separated properly on smaller screens
• fix: harmonize file paths in error messages, consistently based on WP root directory
• fix: parent filter in media gallery
• code: simplify image editor extension to maintain watermark size and position code only once
• code: simplify various data handovers to/from JS