Show and switch between multiple images

[krin-san]

Addresses #7 adding & changing the following:

• Keep multiple images in memory assigned to tabs numbered from 1 to 9
• Handle Cmd+V or Drag&Drop memorising image for the currently selected tab
• Switch between tabs with Cmd+[1-9] hotkey
• Forget assigned image with Cmd+W hotkey
• Window resizes to original image size only if a single image is manipulated, otherwise only window height changes
• Fixed NSTrackingArea not updating bounds automatically
• Fixed a couple of issue with labels incorrectly fading in/out
• Fixed barely visible size label in Dark mode

ℹ️ "Tabs" mentioned here are not conventional AppKit document/window tabs