High bit-depth images: implement linear tone-mapping

Related to #149.

This new code allows PD to import a high bit-depth image without
tone-mapping, which FreeImage does not natively support for 96bpp RGBF
images.  This means we can now import/export HDR images without them
changing!

Still to-do is a screen to present this option to the user.
Fix save state behavior for images imported via URL
Radio button UC: minor fixes
Command bar UC: provide override for "unload parent on OK/Cancel" behavior

Dialogs need this, because their wrapper functions need to retrieve
properties from the dialogs prior to unloading them.  While VB will
leave the dialogs in memory despite being unloaded (meaning the prior
version worked), it also caused two _Load events to be fired, which
causes things like repeat preview refreshes and reduced performance.

Also, the behavior totally crashed my forthcoming high bit-depth import
dialog, which is how I stumbled upon fixing it. :/
HDR and RAW images: display tone-mapping dialog on import

This is a huge precursor to proper high bit-depth image support.  Many
thanks to @hansnolte for providing the impetus to get this implemented!

High-dynamic range images (which come in a variety of formats: RAW, HDR,
EXR, possibly PNG and TIFF) must be downsampled before they can be
displayed on a standard 32bpp monitor.  This process is called
"tone-mapping", and if done incorrectly, the HDR image will look funny
when viewed on screen.

Previously, PD automatically tone-mapped all incoming HDR images.  This
worked well for dSLR imports, but it worked less well for images that
have already been normalized properly.  (Such as HDR images exported
from PD itself!)

As of this commit, incoming high bit-depth images will now trigger a
proper tone-mapping dialog.  This gives the user some control over how
the incoming image is downsampled.  Because there is no way to know in
advance if tone-mapping is required, I think this is the lesser of many
evils.

Images that have already been normalized (such as images exported from
PD) can be imported using a simple linear conversion.  Images from dSLRs
or other sources will need a fancier tone-mapping conversion.  If the
user only ever works with a particular type of HDR image, they can
choose to always use the same tone-mapping settings in the future,
sparing them having to interact with the dialog on every import.

Some features still need to be implemented.  (For example, the "manual"
option is currently just a dummy placeholder.)  But this is a good
start, and I believe it fixes #149 as well.