feat(MediaViewer): improvements

[GeopJr]

• More natural zooming on images with uncommon aspect ratios (/ dont jump-cut to zoomed version on first step)
• Increase button sizes to match Loupe
• Pinching shouldn't append to current size but rather the initial one
• Spinner while image is loading now has a background
• Rethink the way zoom works - instead of having a "scale", (ab)use the scrolled window's adjustments to figure out the sizes, whether it can zoom etc
• Double click zoom in/out
• Allow not expanding picture by default in media viewer

fix: #264
fix: #369
fix: #378