Please test: parallel image loading in geocaches and trackable descriptions

While images were loaded in parallel in the images tab, the way we handled description was:
- display the description without any image;
- fetch the images through an `Html.ImageGetter` then redisplay the description with them.

This way, we could display a partial description if there were problems to access images on the network. Also, the `HtmlImage` getter implemented only serial loading, as expected by Android's `fromHtml()`.

Now, the `HtmlImage` getter can take an optional `TextView`. In this case, images are first replaced by dummy drawables and loaded in the background in parallel. As soon as an image is available, it gets displayed and the `TextView` is invalidated (using its `getText()` method) so that the version with the image is redrawn. This way, description images are shown as they are loaded, and a network stall for an image will not block anything. I have removed the pre-simpler-html display since it is of no use anymore.

Could some people please test this PR and make sure that it looks ok?