This commit standardizes the various todos around the code a bit in a manner recognized by LDoc.
Besides drawing more attention by being displayed in the developer docs, they're also extractable with LDoc on the command line:
```sh
ldoc --tags todo,fixme *.lua
```
However, whether that particular usage offers any advantage over other search tools is questionable at best.
* and some random beautification
This should fix the most obvious memory leaks (noticable when
using images as screensaver).
ImageWidget: added a few more options needed by ImageViewer,
and removed 'overflow' option as logic was wrong.
