Previously the option created by `make_help_action()` was unclear in its
meaning, by renaming the option to 'Manual' this should more
meaningfully represent the effect of the action.
This matches the likes of the adopt_{own, ref}_if_nonnull family and
also frees up the name to allow us to eventually add OOM-fallible
versions of these functions.
The rotate clockwise/rotate counterclockwise actions can be added to
CommonActions since they are repeated in FontEditor, ImageViewer and
PixelPaint. This keeps the shortcuts and icons consistent across
applications.
