Implement basic image viewer popup

Current features:

- Show scaled up thumbnail while the full image is loading
- Click to alternate between scaling mode (or reset zoom if not 1x)
- Click outside of image to close
- Double click to toggle fullscreen
- Middle click to open externally (also for thumbnail in timeline)
- Right click anywhere to close
- Ctrl+wheel to zoom
- Click-drag to pan when image larger than window

TODO.md

@@ -1,6 +1,14 @@
 # TODO
 
+- Image viewer: 
+    - open externally in context menu in timeline thumbnail
+    - fix: gif always closes?
+    - hflickable support kinetic scrolling disabler and speed/decel settings
+    - buttons 
+    - keyboard controls
+
 - Avatar tooltip can get displayed in front of presence menu
+- Use loading cursorShape
 
 - global presence control