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

src/gui/Base/HFlickable.qml

@@ -9,5 +9,6 @@ Flickable {
 
     ScrollBar.vertical: HScrollBar {
         visible: parent.interactive
+        z: 999
     }
 }