update HTML landing page

The original landing page was pretty bad, not just from the looks, but
also from the functionality: In order to paste an image, you had to have
the cursor focussed in the small div.

This edition brings several improvements:

* The general style of the page has been improved by adding some CSS
  styling to adjust the colors and center the content.

* Pasting an image will not work everywhere on the page without
  focussing a specific element. This seems to work because
  document.onpaste fires every time, not just when the element is
  focussed and it even works without contenteditable=true, which removes
  the ugly blinking cursors.

* There is a preview window for pasted images, clearly indicating what
  is going on and which image is going to be pasted.

* Pasting an image will now hide the file chooser.

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