Previously both needed to be specified (unless the image was
being resized to be smaller than its original size).
If height but not width is specified, we now set width to
textwidth (and similarly if width but not height is specified).
Since we have keepaspectratio, this yields the desired result.
- Export `inEm` from ImageSize [API change].
- Change `showFl` and `show` instance for `Dimension` so
extra decimal places are omitted.
- Added `Em` as a constructor of `Dimension` [API change].
- Allow `em`, `cm`, `in` to pass through without conversion
in HTML, LaTeX.
Closes#3450.
